Manually fixing the match by searching for tvdb-95991 worked for me with some test files:
Assuming you’re using the ‘Plex Series’ agent, adding the tvdb id directly to the folder name would have probably avoided the manual match as well (TV/Buck Rogers (1939) {tvdb-95991}/Season 01/Buck Rogers - S01E01.ext).
Also make sure the show’s episode ordering is set to TheTVDB (Edit show > Advanced > Episode ordering).
I don’t know why it’s limited to 8 parts (my guess is some legacy behavior that isn’t a priority to update), but IMO it’s always better to merge the parts together if possible.
