When saving tags to a .m4a (AAC or Apple Lossless) file, the following tags look like they're being added, but if you save and then reload the file, they're not present:
Original Release Date
Performer (including all sub-types)
Picard also indicates that these files are up-to-date even when these tags are missing (i.e. shows a checkmark instead of the green rectangle next to the track.
From what I can tell, the other tags seem to be OK, including similar ones like Composer, Lyricist, Producer, Mixer, DJ-Mixer, and Remixer.
I'm not sure if the bug here is that these tags should be supported but aren't being written properly, or that they're not supported and Picard shouldn't try to write them.
Does not occur with mp3 or FLAC files.