Here is a strange effect I noticed whilst looking at forum member Herchi's claim that Mp3Tag did not delete the Track Count tag
(here). (I'm too late to prevent them posting on the Mp3Tag forum.)
When Track X of Y is tagged, this is stored in the Track# field (tag code TRCK):

If this tag is deleted using the Tag Inspector, the Track# is deleted from the MusicBee file, but not the Track Count:

The same result is seen when deleting the Track# only from the MusicBee edit window.
The same result is seen when using Mp3Tag to delete all tags from the file (and then rescanning in MusicBee) - the Track Count remains but it does not appear to be stored in a tag - I am assuming that it must still be stored in the MusicBee database.
The only way to remove the Track Count data is to delete it using the MusicBee edit window.
(Not sure that this is a massive issue - it has never been an issue for me - but thought I would report it anyway.)
Note: With vorbis comments this is not an issue as the Track Count is stored in a different tag from Track#.
MusicBee 3.6.8954P