I think disc/track numbers may be the issue, especially since it appears that the tracks for all albums are in a single, flat folder. Until recently, having albums in a flat folder, rather than an artist/album/tracks structure, caused major problems for Plex. A recent update fixed that, but only if embedded tags are in really good order.
I’d try adding disc/track numbers to one album that’s currently split up, rescan the library, and see what happens. Might have to Plex Dance.
I’m guessing Plex won’t understand ‘A1’ ‘B1’ track numbers. Try just 1 through 10 (if there are 5 ‘A’ and 5 ‘B’ tracks) as an experiment.
