I know the vertical tag editor supports customization of fields, although not sure if it's to that extent.
MusicBee should be changing the files themselves, so not sure what would be going on. You can check the actual tag content of the files using the Tag Inspector.
Is the album artist field actually're-populating, pr is it just treating the artist as the album artist? That's expected behavior, unless you have the compilation flag set and an album artist defined in sorting/grouping preferences.