Because the files in your regular and your 4K library are ultimately the same movie.
If both libraries use the same metadata agent, then both files get the same ‘ID number’ (which corresponds to the ID of this movie on the IMDb).
If two files have the same ID, they are the same movie.
That’s why Plex is offering you both files in Continue Watching.
A workaround is to use different agents (‘Plex Movie’ vs. ‘TMDB’) for your two movie libraries, so that the ID number is totally different, even if the same movie is on both libraries.
