have been using for years, updated for the first time in a while from 2.91 to 2.97, later on noticed that some tags created in older files do not show anymore. After a while of doubting my encoding (dbpoweramp) and re-entering tags without progress and NOT DOUBTING MP3TAG cos it has always been "OK", downgraded back to 2.91 and all tags are there.
Trouble is I only thought of downgrading after re-encoding and RE-Entering many hundreds of albums. What a waste of time..
You could have checked the settings in File>Options>Tags>Mpeg in respect to APE tags.
Also, you could check your files if you have APE tags with the filter
%_tag% HAS APE.
As APE tags overrule the display of other tags, it could be that empty APE fields hide the filled ID3V2 tag fields.
I didnt expect any changes were made to the settings! but I know now..
Usually the settings are kept.
But if you choose a different way of installation (normal vs. portable), install a portable version in a different folder than before, the settings are reset to default.
So I would check the files for APE tags as apparently they are out of sync with the other tags.
The best way would be to remove APE tags altogether.