ReplayGain tags from rsgain in .m4a files are ignored

[dseomn]

Steps to reproduce

1. Run rsgain on a .m4a file: rsgain custom --album --tagmode=s --clip-mode=n foo.m4a
2. Re-scan library.
3. Select the file and edit tags.

Expected Output

ReplayGain tags should show up.

Actual Output

Neither Quodlibet nor Exfalso show replaygain tags, but mutagen-inspect shows:

...
- MPEG-4 audio (AAC LC), 202.04 seconds, 125588 bps (audio/mp4)
...
----:com.apple.iTunes:REPLAYGAIN_ALBUM_GAIN=MP4FreeForm(b'-4.99 dB', <AtomDataType.UTF8: 1>)
----:com.apple.iTunes:REPLAYGAIN_ALBUM_PEAK=MP4FreeForm(b'1.000000', <AtomDataType.UTF8: 1>)
----:com.apple.iTunes:REPLAYGAIN_TRACK_GAIN=MP4FreeForm(b'-4.99 dB', <AtomDataType.UTF8: 1>)
----:com.apple.iTunes:REPLAYGAIN_TRACK_PEAK=MP4FreeForm(b'1.000000', <AtomDataType.UTF8: 1>)
...

Test System

Which version of Quod Libet?

4.6.0-1

Which operating system

Debian testing

If it's audio-related, what back-end?

N/A

Additional Information

mutagen-inspect on another m4a file where I used something else to generate replaygain tags (exfalso I think maybe?) shows:

----:com.apple.iTunes:replaygain_album_gain=MP4FreeForm(b'-4.90 dB', <AtomDataType.UTF8: 1>)
----:com.apple.iTunes:replaygain_album_peak=MP4FreeForm(b'1.0080', <AtomDataType.UTF8: 1>)
----:com.apple.iTunes:replaygain_track_gain=MP4FreeForm(b'-4.90 dB', <AtomDataType.UTF8: 1>)
----:com.apple.iTunes:replaygain_track_peak=MP4FreeForm(b'1.0080', <AtomDataType.UTF8: 1>)

So maybe the issue is upper-/lower-case difference?

[declension]

QL specifically looks for

----:com.apple.iTunes:replaygain_album_gain
----:com.apple.iTunes:replaygain_album_peak
----:com.apple.iTunes:replaygain_track_gain
----:com.apple.iTunes:replaygain_track_peak
----:com.apple.iTunes:replaygain_reference_loudness

and does so case-sensitively, so I guess this is as expected. Whether that's truly correct, I don't know.