This isn't much to go on, I know. But.
I have a multi-disc (55 discs) album, where all tracks show the same data for "album". Using Mp3tag, I modified two discs to change "album" to "discnumber.album". This is to facilitate playback via MS's new Media Player, which does not sort by disc number, only by track number within album, showing me 55 tracks #1, for example.
All is good except that track #1 sorts to the bottom of the track list for both discs using MS's new Media Player. Other albums that are unmodified sort correctly. I have checked "track" carefully, since Media Player seems to sort by track within album. All other visible tags are correct.
The old Windows Media Player sorts correctly, and so does File Explorer.
Is it possible that there is a hidden tag involved here, written by Mp3tag, that Media Player uses, but cannot be seen?
I have a similar question in to Microsoft.