I'm using mp3tag (always latest dev version, 2.46 currently) in my digital import process (EAC->M4A (AAC) using iTunesEncode) to tag all my aac/mp4 library, but it seems there is some problem with the mp4 tags written by mp3tag with Android (the mobile OS):
taking a cleanly converted WAV->M4A by itunes and tagging it with mp3tag, the tags are not read at all by android.
opening this file in itunes shows correct information, letting itunes rewrite the tag by changing a flied (for example the compositor field) makes the file works perfectly on android.
If I rewrite the tag again in mp3tag, the tags become unreadable to android again.
It seems there's definitely something with the way mp3tag handle the mp4 tags, unfortunately, I can't provide much more informations on this :-/
You could upload two versions of the same track ("after Mp3tag" and "after iTunes") say to drop.io and post the link here.
Then there should be enough details for the problem analysis.
The first file is the one just passed through mp3tag after the extraction.
The second file was then went through itunes and has some tags modified then reverted to force rewrite of the mp4 tags.