You should check how you treat APE tags (Tools>Option>Mpeg).
It could be that MM needs the APE tags - so you have to read, write and delete them too.
Or you try a ID3-ony-apporoach and remove and ignore the APE tags altogether and see what MM does.
Please be more specific about what kind of tags you are talking?
I sometimes use MM too for special tasks and my tags, created with MP3Tag, are always showing in MM.
MM Library Options "Update file info from Tags when rescanning files" was UNCHECKED for some reason. It could have been years ago but since I never used any other tag management program before except MM, it made no difference.
I ticked the box, rescanned the folders. All changes made with mp3tag were reflected correctly.
Thank you to everybody who offered assistance. Much appreciated.