Official comment
See this chart as to which files bring in which metadata on import.
See this chart as to which files bring in which metadata on import.
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Check the actual name of the ID3 tag with kid3. Rekordbox only reads the "key" tag for M4A files, but some other programs write the key to the "initialkey" tag.
Using kid3 helped, as I discovered that the tag editors I was using were not actually tagging the correct fields - even the one where I can specifically type a field name in e.g. entering "key" as the tag to update instead of "initialkey" was still just only updating the "initialkey" tag.
However, another issue popped up with using kid3 and that was m4a files becoming corrupted. Not exactly sure what was happening, but a large chunk of my files randomly lost tags & became unable to be played. I replaced them and tried again, then carried on & the same thing happened to other m4a files.
I decided to cut my losses & that tagging apps (I was using 5 at one point trying to fix this) are not ready for advanced tagging on m4a/aac files, so I reverted back to using mp3s...
In short, if it ain't broke, don't try to fix it!