It would be great to have the ability to convert or add ID3v tags to MP4 files. (I have some players that, amazingly, will play .m4a files, but can't read MP4 tags, or some characters within the MP4 tag).
I believe that MP4 supports ID3v tags (https://mp4ra.github.io/mp4ra/specs.html).
I think that if you set ID3V2.3 tags (is this what you mean?) on top of MP4 tags, you get an invalid format.
I doubt that MP3tag will ever be modified to write an invalid format.
Perhaps you have to check Tools>Options>Tags>Mpeg and see what character code you write (although I am not even sure that this can be changed for MP4s)