Other sources like Wikipedia tell us that iTunes is more or less the only widespread software that implemented id3 tags for MP4 and the implementation has the limitation to 255 characters per (text) field.
So if Aplle calls this limitation a "feature" I would be reluctant to call it a bug in MP3tag.
If you use MP3 files, you do not have this limitation, as you quite rightly mentioned.
If iTunes read only 255 characters, so be it. mp3tag could write the id3tag with no limit, this doesn't affect itunes.
I have m4a with id3tags, windows media player can read the tags. I write only id3tag v2.3 to all my files. So, other players can read the id3tags with m4a.
Please remove field limit for id3tag with m4a. If not for id3tags support by itunes (artist, album,...) at least remove the field limitation for other user defined fields, that user can store necessary data with m4a.
Eventually, apple decides to remove the field limit, user can move over the data to standard fields.
Sorry, I am not here to discuss, if this is a bug or not? if post should be posted in general discussion sub-forum or bug sub-forum?
I want to point out something I noticed. If it offends you, issue being called a "bug", I can't help much. Last I heard, no software in this world is ever free bug, but conditionally stable and satisfactory!