The issue you're seeing is because I misused the list.Add() function in my previous example, sorry for that. Namely, the arguments are different, from how I used them. If you like to have priority "Expression 10", you'd need to use the following line:

list.Add(op, c4d.EXECUTIONPRIORITY_EXPRESSION + 10, c4d.EXECUTIONFLAGS_NONE)

I've updated the script in my previous posting with the proper usage of the list.Add() function.

Please refer to an adjacent thread regarding execution flags: Priority: PriorityData and PriorityList

Cheers,
Ilia

With the code snippet below and Cinema 4D 2025.2.0 I cannot reproduce your issue. Namely the python tag with Expression-0 and the object modifier with Expression-10 work as expected:

Cheers,
Ilia

Python tag code:

import c4d
def main() -> None:
    print('tag')

stemp.pyp code:
Upd 14/04/25: fixed the list.Add() function

import c4d
PLUGIN_ID = 1051592
class S_Temp(c4d.plugins.ObjectData):
    def Init(self, node, isCloneInit: bool) -> bool:
        print('stemp-Init')
        return True
    
    def AddToExecution(self, op, list) -> bool:
        print("stemp-AddToExecution")
        list.Add(op, c4d.EXECUTIONPRIORITY_EXPRESSION + 10, c4d.EXECUTIONFLAGS_NONE)
        return True
    
    def Execute(self, op, doc, bt, priority, flags) -> int:
        print(f"stemp-Execute: {priority}")
        return c4d.EXECUTIONRESULT_OK

    def ModifyObject(self, mod, doc, op, op_mg, mod_mg, lod, flags, thread):
        print(f"stemp-ModifyObject: {op.GetName()}")
        allp = [pos + c4d.Vector(0, 100, 0) for pos in op.GetAllPoints()]
        op.SetAllPoints(allp)
        op.Message(c4d.MSG_UPDATE)
        return True

if __name__ == '__main__':
    c4d.plugins.RegisterObjectPlugin(
        id=PLUGIN_ID,
        str="S_Temp",
        g=S_Temp,
        description="stemp",
        info=c4d.OBJECT_MODIFIER | c4d.OBJECT_CALL_ADDEXECUTION,
        icon=None
    )

my code:

    def Execute(self, op, doc, bt, priority, flags) -> int:

        print("Execute")
        return c4d.EXECUTIONRESULT_OK

    def AddToExecution(self, op, list) -> bool:
        print("AddToExecution")
        mode = op[c4d.S_TEMP_PRIORITY].GetPriorityValue(c4d.PRIORITYVALUE_MODE)
        list.Add(op, mode,c4d.EXECUTIONFLAGS_NONE)

Sorry for the late answer and thanks for providing a great explanation of your question with the code snippet and a sample plugin, highly appreciated!

I suspect, the missing bit in your case is the c4d.OBJECT_CALL_ADDEXECUTION flag that you need to add for your object plugin registration function call.

However, this will allow you to structure the sequence of calling the Execute() function. You cannot queue the execution of the ModifyObject() function.

Cheers,
Ilia