When posting code, I also recommend being sure that it's reduced to the bare minimum to reproduce the issue, that's properly documented and that a scene using it is attached to help us reproducing the issue rather than guessing what to do.

Riccardo

class CloneStalker (plugins.ObjectData):
def init (self):
self.SetOptimizeCache(True)

def Init(self, node):
    # Parameter initialization
    self.InitAttr(node, float, [c4d.CL_STALKER_MAX_NULLS])

    node[c4d.CL_STALKER_MAX_NULLS] = 100.0


    pd = c4d.PriorityData()
    if pd is None:
        raise MemoryError("Failed to create a priority data.")

    pd.SetPriorityValue(c4d.PRIORITYVALUE_MODE, 4)
    pd.SetPriorityValue(c4d.PRIORITYVALUE_PRIORITY, 100)
    node[c4d.EXPRESSION_PRIORITY] = pd


    return True


def CheckDirty(self, op, doc):

    frame = doc.GetTime().GetFrame(doc.GetFps())
    if frame != lastFrame:
        lastFrame = frame
        op.SetDirty(c4d.DIRTYFLAGS_DATA)

def AddToExecution(self, op, list) :  
    list.Add(op, c4d.EXECUTIONPRIORITY_GENERATOR, 400)  
    return True  



def GetVirtualObjects(self, op, hh):
    cloner = op[c4d.CL_STALKER_TARGET]
    doc = c4d.documents.GetActiveDocument()

    def GetStalkers(bname,tgt):
        allChild = tgt.GetChildren()
        stalkerChilds = []
        if allChild != None:
            for i in allChild:
                childname = i[c4d.ID_BASELIST_NAME]
                bnamelenght = len(bname)
                subname = childname[0:bnamelenght]
                if subname == bname:
                    stalkerChilds.append(i)

        return stalkerChilds

    if cloner != None:
        modata=mo.GeGetMoData(cloner)
        if modata != None:
            cache =  cloner.GetDeformCache()
            if cache is None:
                cache = cloner.GetCache()
            clones = cache.GetChildren()
            name = str(cloner[c4d.ID_BASELIST_NAME]) + ".Cloner Stalker"
            child_basename = op[c4d.ID_BASELIST_NAME].replace(".Cloner Stalker", ".Stalker.")

            if cloner.GetNext() != op:
                op.InsertAfter(cloner)
            null= c4d.BaseObject(c4d.Onull)
            null[c4d.NULLOBJECT_DISPLAY]=0
            null[c4d.ID_BASELIST_NAME] = name
            parent=op
            

            md = mo.GeGetMoData(cloner)
            marr = md.GetArray(c4d.MODATA_MATRIX)
            count = md.GetCount()
            Selection = []
            maxNulls = op[c4d.CL_STALKER_MAX_NULLS]
            lastChild = None
            child = GetStalkers(child_basename,op)
            if mo.GeGetMoDataSelection(cloner) == None:
                for i in range(count):
                    Selection.append(i)
                selected=Selection
            if  mo.GeGetMoDataSelection(cloner)!= None:
                Selection = mo.GeGetMoDataSelection(cloner).GetAll(count)
                selected=[i for i,x in enumerate(Selection) if x==1]

            if op[c4d.ID_BASELIST_NAME]!=name:
                child_basename = op[c4d.ID_BASELIST_NAME].replace(".Cloner Stalker", ".Stalker.")
                if len(child)>0:
                    for i in child:
                        old_name =i[c4d.ID_BASELIST_NAME]
                        index_name = old_name[len(child_basename):len(old_name)]
                        n_name = cloner[c4d.ID_BASELIST_NAME] + ".Stalker." + index_name
                        i[c4d.ID_BASELIST_NAME] = n_name
                        print (n_name)
                
                op[c4d.ID_BASELIST_NAME] = name
                child_basename = op[c4d.ID_BASELIST_NAME].replace(".Cloner Stalker", ".Stalker.")
                child = GetStalkers(child_basename,op)    
                parent=op
            op.SetMg(cloner.GetMg())

            if len(child)<len(selected) and len(child) < maxNulls:
                currChilds=[]
                for i in child:
                    child_name = i[c4d.ID_BASELIST_NAME]
                    nameLenght = len(child_name)
                    index = int(child_name[len(child_basename):nameLenght])
                    currChilds.append(index)
                new = [item for item in selected if item not in currChilds]
                for i in new:
                    obj= c4d.BaseObject(c4d.Onull)
                    obj[c4d.ID_BASELIST_NAME] = child_basename + str(i)
                    obj[c4d.NULLOBJECT_DISPLAY]=3
                    obj[c4d.NULLOBJECT_RADIUS]=50
                    extCompTag = c4d.BaseTag(465000402)
                    matrix = clones[i].GetMg()
                    obj.SetMg(matrix)
                    obj.InsertTag(extCompTag)
                    if len(child)>0:
                        lastChild = child[(len(child)-1)]
                    doc.InsertObject(obj,parent,lastChild)
                    child = GetStalkers(child_basename,op)
                    if len(child)==maxNulls:
                        break
            if len(child)>len(selected) or len(child)>maxNulls:
                for i in child:
                    child_name = i[c4d.ID_BASELIST_NAME]
                    nameLenght = len(child_name)
                    index = child_name[len(child_basename):nameLenght]
                    if not int(index) in selected:
                        i.Remove()
                    child = GetStalkers(child_basename,op)
                while len(child)>maxNulls:
                    lastChild = child[(len(child)-1)]
                    lastChild.Remove()
                    child = GetStalkers(child_basename,op)
                    
           
               
def Execute(self, op, doc, bt, priority, flags) :
    
    def GetStalkers(bname,tgt):
        allChild = tgt.GetChildren()
        stalkerChilds = []
        if allChild != None:
            for i in allChild:
                childname = i[c4d.ID_BASELIST_NAME]
                bnamelenght = len(bname)
                subname = childname[0:bnamelenght]
                if subname == bname:
                    stalkerChilds.append(i)

        return stalkerChilds
    cloner = op[c4d.CL_STALKER_TARGET]
    doc = c4d.documents.GetActiveDocument()
    name = str(cloner[c4d.ID_BASELIST_NAME]) + ".Cloner Stalker"
    child_basename = op[c4d.ID_BASELIST_NAME].replace(".Cloner Stalker", ".Stalker.")
    parent=op
    child = GetStalkers(child_basename,op)
    Selection = []

    if cloner != None:
        modata=mo.GeGetMoData(cloner)
        if modata != None:
            cache =  cloner.GetDeformCache()
            if cache is None:
                cache = cloner.GetCache()
            clones = cache.GetChildren()
            md = mo.GeGetMoData(cloner)
            marr = md.GetArray(c4d.MODATA_MATRIX)
            count = md.GetCount()

            if mo.GeGetMoDataSelection(cloner) == None:
                for i in range(count):
                    Selection.append(i)
                    selected=Selection
            if  mo.GeGetMoDataSelection(cloner)!= None:
                Selection = mo.GeGetMoDataSelection(cloner).GetAll(count)
                selected=[i for i,x in enumerate(Selection) if x==1]

            for x in selected:
                matrix = clones[x].GetMg()
                ch_name = child_basename + str(x)
                obj=c4d.documents.BaseDocument.SearchObject(doc,ch_name)
                obj.SetMg(matrix)
                print (clones[x].IsDirty(1))
    return c4d.EXECUTIONRESULT_OK        

Please feel free to advise what i am doing wrong here, and what shuold i be doing instead.

many thanks for your patience.
cheers
Eddie

With regard to your issue, I think the problem is how you handle your ObjectData cache and when if update it during the scene evaluation. In the code below beside checking for my object cache existence and for my object's parameters changes I also check if the cloner's cache has changed when the ObjectData::GetVirtualObjects is called.

class PC_12764 (plugins.ObjectData):
	_clonerDirty = 0

	def Init(self, node):
		return True

	def GetVirtualObjects(self, op, hh):
		cloner = op[1002]
		res = c4d.BaseObject(c4d.Onull)
		if cloner is None:
			return res

		clonerCache = cloner.GetCache()
		if clonerCache is None:
			return res

		firstClonerItem = clonerCache.GetDown()
		if firstClonerItem is None:
			return res

		# check cloner cache dirty count
		clonerDirty = False
		if self._clonerDirty != cloner.GetDirty(c4d.DIRTYFLAGS_CACHE):
			clonerDirty = True
			self._clonerDirty = cloner.GetDirty(c4d.DIRTYFLAGS_CACHE)
		
		# return object cache taking in account cloner cache
		dirty = op.CheckCache(hh) or op.IsDirty(c4d.DIRTY_DATA) or clonerDirty
		if not dirty :
			return op.GetCache(hh)
		
		while firstClonerItem is not None:
			child = c4d.BaseObject(c4d.Onull)
			child.SetMg(firstClonerItem.GetMg())
			child.InsertUnder(res)
			firstClonerItem = firstClonerItem.GetNext()

		return res

This approach, without using the ObjectData::Execute() gives ,in the end, consistent results when exporting a .aec file using a External Compositing Tag as in the scene attached.

AEC file except

...
...

NULL2 "Null"
{
  NULLTYPE 1
  SIZEX 200
  SIZEY 100
  ANCHOR 3
  ORIENTATION 1
  COLOR 1 0 0 
  SHOWFROM 1
  SHOWTO 10
  KEY 1 200 200 200 0 0 0 0 0 0
  KEY 2 202.036 195.61 204.755 0 0 0 0 0 0
  KEY 3 206.929 185.058 216.184 0 0 0 0 0 0
  KEY 4 212.786 172.427 229.866 0 0 0 0 0 0
  KEY 5 217.679 161.875 241.295 0 0 0 0 0 0
  KEY 6 219.714 157.485 246.051 0 0 0 0 0 0
  KEY 7 217.679 161.875 241.295 0 0 0 0 0 0
  KEY 8 212.786 172.427 229.866 0 0 0 0 0 0
  KEY 9 206.929 185.058 216.184 0 0 0 0 0 0
  KEY 10 202.036 195.61 204.755 0 0 0 0 0 0
}

NULL2 "Null"
{
  NULLTYPE 1
  SIZEX 200
  SIZEY 100
  ANCHOR 3
  ORIENTATION 1
  COLOR 1 0 0 
  SHOWFROM 1
  SHOWTO 10
  KEY 1 0 200 200 0 0 0 0 0 0
  KEY 2 0.427 201.541 196.343 0 0 0 0 0 0
  KEY 3 1.453 205.246 187.554 0 0 0 0 0 0
  KEY 4 2.681 209.681 177.032 0 0 0 0 0 0
  KEY 5 3.707 213.385 168.242 0 0 0 0 0 0
  KEY 6 4.134 214.927 164.585 0 0 0 0 0 0
  KEY 7 3.707 213.385 168.242 0 0 0 0 0 0
  KEY 8 2.681 209.681 177.032 0 0 0 0 0 0
  KEY 9 1.453 205.246 187.554 0 0 0 0 0 0
  KEY 10 0.427 201.541 196.343 0 0 0 0 0 0
...
...
}

Scene file

PC_12764.c4d