Sure this is as it is ... Mp3tag works with tag data but never with the music stream data.
One can apply MP3Gain in non destructive mode (?), ...
and write ID3 tag, ... see MP3gain command line syntax description, you can find it already here on the forum board anywhere.
DD.20160506.0857.CEST
See also ...
Mp3Gain DOS commandline tool help:
mp3gain.1.5.1.help.txt ( 3.71K )
https://forums.mp3tag.de/index.php?act=atta...ost&id=2768
einheitliche Lautstärke bei mp3s
https://www.google.de/search?q=site%3Aforum...gain+id3+%2Fs+i
DD.20160510.1053.CEST