Additional test on the same starting file:
I used ffmpeg -map_chapters -1 to remove the chapter information when making a copy, and then make the same change to the tags.
The Tag field still displays MP4 (MP4). In other words, the Nero tags are NOT added if there is NO chapter information, and the Nero tags ARE added if there IS chapter information.
Is there any way to suppress the addition of the Nero tags? Clearly they aren't necessary in this file, and it is only the presence of chapter information that triggers their creation when editing the existing MP4 tags.
By "Apple gapless data" are you referencing the legacy ITUNESGAPLESS pgap field that needs to be set to 1? While I don't believe current versions of iTunes specifically uses this any longer, it can still be used and set in mp3tag if that is what is required for keeping the chapters from getting generated.
In the case of the OP's concern, is the indication of "MP4 NERO" just a change of format based on how mp3tag handles chapters in m4a? I don't think there are other specific tags added, just the format change, correct?
Yes, this is how it's implemented at the moment. I went that route to be most compatible with foobar2000, which is using a similar approach (at least at the time). It's basically a representation of the chapters in Nero form — your original chapters are still there.
No, it's currently not possible.
This was a different issue that has been fixed with v2.93a. Back then, Mp3tag created the chpl atom in that case no matter if chapters were present or not.
[2019-01-29] CHG: chpl atom was created if Apple gapless data was present. (#44323)
It's an addition of the Nero representation. No change of the existing format.