I can’t imagine I’m in a tiny minority by tagging .m4a files with Picard, but it’s giving me a lot of trouble. Part of it is related to Picard 1098 and the fact that custom tags aren’t shown or saved for m4a files. (Which originated with this post 2 years ago.)
It also appears that performers just aren’t tagged for m4a - is that correct?
I’m still trying to sort out some kind of issue where the file shows correct artists in Picard, but MusicBee can’t see an artist value at all. Sometimes it’s fine but it’s happening consistently enough that I need to figure out what’s going on. It also seems like in these cases Picard is removing custom tags even without “clear existing tags” checked. If I can narrow down steps, I’ll make a ticket for it, but for now just seeing if anybody else has run into these kinds of troubles and if there’s anything I can do about it.
No, there seems to be no standard way to doing this. But if there is a tagging commonly supported by players we could add it.
You can always see what tags Picard writes for which format at Appendix B: Tag Mapping — MusicBrainz Picard v2.11.0rc1 documentation
If you could provide an example file I could check it. But I think you should start asking the MusicBee developer about that first. There is no source code available for MusicBee, so we can only guess what it is doing.
No, Picard will never remove tags unless you explicitly delete them or you have “clear existing tags” active.
But last time I checked MusicBee did not support writing tags to MP4 files, so my suspicion would be that the tags are not inside the files at all.
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MusicBee has no trouble writing to m4a files (including custom tags) or reading m4a tags as written by iTunes. I’ll bring it up with MusicBee when I can nail down where the problem is happening, but since Picard is having more trouble with m4a in general I’m not going to assume that MusicBee is the problem right off the bat (I’m not assuming that it’s Picard, either, but it’s only after tagging with Picard that I’m seeing odd results).
My memory might have been a bit muddy or outdated, maybe I confused it with APE tags. But I tested, and Picard here does not remove custom tags (inlcuding performer tags) saved with MusicBee per se. But if you save a tag with multiple values with Picard it becomes invisible to MusicBee. My guess is that this is related to the way Picard stores multi-value tags, see PICARD-376.
I also cannot find any documentation for MP4 tags in MusicBee. Maybe someone with more knowledge of MusicBee’s internals should be bugged to update their tagging docs 
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That fits with what I’m seeing, yeah. I may be mistaken about the custom tags, it was particularly MusicBee’s Display Artist that I noticed, which could be related to the Artist problems.
That would probably be me. I wrote most of the current wiki content, including that page.
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