Are you 100% sure all the files are in the same folder, and there is not (even) one in another folder?
Also, if you care about having proper metadata, and have put in the effort for that, consider not using 'files for each album are organised in their own folder'?
Not saying that is related to this particular issue, but MusicBee is intended and optimised to be used using proper metadata.
Making folder locations relevant to grouping might complicate things in respect to just being able to rely on your metadata.