After upgrading to v2.49 from v2.48 I noticed that Mp3tag wasn't reading the ID3 tags of some of my files. This seems to occur after running mp3gain. I was trying to keep all my id3 tags v2.4, mp3gain changes the tags to v2.3 so I use mp3tag to set them back to v2.4. This worked until mp3tag v2.49, when I saved to the new format all my id3tag info was gone, I restored my files from a backup and repeated the process, but this time I noticed that mp3tag had nothing in the tag columns after mp3gain, went to another tag editing program and saw that the information was still there, reverted to mp3tag v2.48 and loaded the files again and everything was ok. Nasty bug if you don't keep backups.
It seems, that you are stumbled over a known problem situation when applying the MP3Gain utility in default mode writing APE tag.
Check the Mp3tag settings for "Mpeg" and understand the priority how different tag types will be used within Mp3tag.
Use mp3gain.exe to be compatible with the ID3 tagging system.
Better use foobar2000 to write non destructive ReplayGain values into ID3 tag fields.
Find informations there ...
Anzeigeformat in Spalte "Tag" (revisited)
.. and there ....