I've looked at some of the other topics regarding tagging MP4 files and it's never clear to me whether they are referring to MP4-type tags for MP4 files, or all tag types for MP4 files. Just to be clear, my issue / question is in regard to MP4-type tags (as opposed to MP3TAG, ID3, iTunes, etc.)-type tags, as defined in this crucial table ( Mp3tag Help - Tag field mappings ).
My limited experience with mapping tags for MP4 files is with ID3v2 tags. They seem to work as advertised. But I have big problems with mapping MP4-type tags. When I use an ID3v2-type tag, the value assigned to the tag via MP3TAG gets attached to the tag, as evidenced by View > Extended Tags…, and it continues to be displayed on the MP3TAG tag panel. In other words, it behaves as you'd expect it to behave.
But when I use at MP4-type tag, the value assigned to the tag via MP3TAG still gets attached to the tag, as evidenced by View > Extended Tags…, but it disappears from the MP3TAG tag panel, never to be seen again.
Specifically: if I map
Tag: IDv3 - Source: COMPOSER - Target: TCOM
all's well. But if I map
Tag: MP4 - Source: COMPOSER - Target: ©wrt
the value assigned via MP3TAG shows up attached to ©WRT in View > Extended Tags BUT it's disappeared from the tag panel and to the best of my knowledge the only way to get rid of it would be to delete the MP4 file
Please note - I'm assuming that the "Tag (type)" field on the left of the mapping table refers to the "Target" field on the right. The center column, "Source" always refers to the MP3TAG name, as I understand it. If that's the case I think the Mapping Table would make more sense if the columns were adjusted such that "Source" came first, then "Tag (type)", and then "Target". And a pull-down list for "Source" would be an outstanding improvement.
I've seen one other topic to that addressed this point but it wasn't as specific as I've tried to be.
As an aside, the whole subject of mapping metadata for MP4 files from MP3TAG all the way through to Plex is a huge can of worms but it can't be addressed until and unless we can present the data to Plex via metadata. There are bound to be instances where that will require the use of MP4-type tags, and I'm thinking of DIRECTOR and WRITER specifically.