<keep> is OK, I think.
Now the last thing I could think of: have you got more than 1 complete tag in the file. (Although then the display in MP3tag should be consistent and show only the data from one tag)
You could test this for one file:
Select it
Select "Cut tag" from the file list context menu
Check if the file now displays not data in the tag fields. If this is the case, then you have only 1 tag. If you still see data and perhaps even the old one, then there was one tag stacked on top of the other.
You can now choose whether you want to keep this data or delete the tag and paste the one that you have in the clipboard.
And then there is the possibility that the files got damaged somewhere which you could check the ujtilities linked in this thread:
And why all this trouble? Because I can write GENRE fields without any problems