Selecting an Actor in my TV-Series library doesn’t search for that actor in my Movies-Library or my other TV-library (another primary language).
Even though the actor is present in titles on all these libraries I only receive results from the currently selected one.
Both are well known issues/limitations of the cast members functionality in plex. Do a quick search of the forum and you will find many many topics on these issues. Feature requests also exist so best to vote on them.