Are you using Custom Tabs, or standard tabs?
I'm tagging using MP3tag. I haven't customised any tabs.
I think it would help to have a specific example. What's a track where a tag was updated but doesn't show up in MusicBee after a rescan? What format it is it? What do you find for that tag in the tag inspector?
I've had an album tagged 'Folk' and yet it still retained its old tag (Folk, World and Country and Pop) in Genre, but I've since deleted it and got it from another source, the problem is no more since they are not, apparently, the same files and I deleted the old ones anyways. I've just found an example, though:
https://i.imgur.com/8L8kbBW.png compare with
https://i.imgur.com/3cqoVSx.png where both Artist and Album Artists are "Ling Tosite Sigure (凛として時雨)" yet the tag is still "Ling tosite sigure" (presumably the old tag it had), this only happens with this one album from "Ling Tosite Sigure (凛として時雨)", even where the others where tagged the same becfore I re-tagged them. There's one that's even worse, here:
https://i.imgur.com/k8mOToQ.png compare with
https://i.imgur.com/fT19v8v.png. Of course, 'LBUM ARTIST' and 'LBUM' were never tags at all (Album Artists tags, apparently), yet they do not disappear even after these albums have been tagged properly. I have no idea why this happened and a file re-scan does nothing. As for format, all my files are MP3 always.