I've got a new hint!
Today I created a Plex Server to have a try when those files are available to MRMC via Plex instead of Emby, once added the actor information was found (and in what other movies the actor is playing as well):
When selecting PLEX as source, it worked.
When selecting Emby as source, it worked.
When removing PLEX and selecting EMBY as source again, it didn't work any longer!
So there is some issue within MRMC I think

.
Would be nice if anyone could look into this.
Thanks