With that's said I think you need to at least execute the scene once with ExecutePasses just after your Message(c4d.MSG_UPDATE).
The reason is because MSG_UPDATE have the purpose to update internal cache of the geometry, and also set dirty the cache (aka say it needs to be re-generated). This is the same with the tags you added it most likely also set dirtiness (needs to be recomputed on the next scene execution).
And then you call EventAdd at the end of your script, so you are telling Cinema 4D that he needs to do another ExecutionPass (Scene Execution) which will be what Cinema 4D will do most likely very soon after the Python Script have been executed. And therefor when you run the script in the console, the scene have been executed, and therefor the tag and everything have been executed and therefor the value are "correct".

Cheers,
Maxime.