Did you re-read your changed tracks in MM after you made any changes in Mp3tag?
MM only detects changes, if you
a) manually re-read this tracks
or
b) set "folder(s) to monitor" for tracks that you change inside such a directory
Mediamonkey normally shows you the content of it's database and not what's inside the tags.
So you habe to rescan your files so that Mediamonky updates it's database.
Have you set the option "Preserve file modification time when saving tags" in Mp3tag?
Disable that and rescan your files in Mediamonkey.
My "music"-folder is set to "scan continously" (because it takes some time to have it scanned completely at each startup), and I have manually "added / re-scanned the library". I even startet Media Monkey new - the changes made with MP3Tag are not shown.
It works perfectly the other way round: If I change tags in MM and refresh MP3Tag, the changes are shown in MP3Tag.
That's because MM works with a database (a collection of previously scanned id3 tags). You could even delete all your songs and still see this tags in MM. Don't do it anyway.
Mp3tag read the tags directly from the tracks every time you load them. So you see every change instantly.
You should update your Mp3tag to the newest version and turn off "Preserve file modification time when saving tags" in Mp3tag. This way Mp3tag changes the last modified date for your changed tracks. This should show MM to re-scan such files.
Update 26.05.2025 for future readers:
You can find the option "Preserve file modification time when saving tags" in newer Versions of Mp3tag in the section:
File -> Options -> General
That's the option in Mediamonkey. I was talking about the option in MP3Tag:
File->Options->Tags
To my opinion it should me unchecked so that other programs know for certain that something in the files has changed.
I had a look at your first post again and you are mentioning "Album Artist".
The ID3v2-tag is called ALBUMARTIST in Mp3Tag. It is possible that you mix them in the 2 programs.
Have a look at all your tags in Mp3Tag with the extended tag-view (ALT-6) and tell us what you see. I suspect that you see 2 or more different tag-fields.