So the problem is that in your files there are multiple values for the “MusicBrainz Release Id” tag. That for sure has its origin in the conversion from FLAC to MP4 using foobar2000.
The problem is that Picard stores e.g. the MB release ID (internal tag name is
musicbrainz_albumid) in a tag called
----:com.apple.iTunes:MusicBrainz Album Id for MP4, but in
MUSICBRAINZ_ALBUMID for FLAC. When foobar2000 converted your files it copied that over into a tag
Now you have files with both tags. According to its own mapping Picard reads the
----:com.apple.iTunes:MusicBrainz Album Id and interprets it as the
musicbrainz_albumid. Then it reads
----:com.apple.iTunes:MUSICBRAINZ_ALBUMID, for Picard this is an unknown tag. But it reads those custom tags with just their name in a case insensitive manner, which means, this also turns out to be
musicbrainz_albumid. And now you have two values.
The one issue causing Picard to fail loading now is that it does not expect multiple values for those tags when loading files, so it fails to load the proper release. I have prepared a fix for this for the next release, see:
The other issue is how Picard should handle clashes between custom tag names and its own tag mapping. We had a similar issue recently discussed here (for MP4 or ID3, I don’t remember, but the problem is similar). I have no good idea. Maybe Picard should just ignore custom tag names if they clash with own names.
Anyway, there is a workaround for your issue. It’s maybe a bit counter intuitive, but you should be able to delete the MB release ID tag from the files, then save them. Then load them again.
If my theory is correct this should result the files to load again with only a single value for the album ID (it will delete the official tag, but leave the custom one in the file). Downside is that with next tagging you will have duplicates again.
The cleaner solution would be to edit the tags, remove the duplicates values from the MBID tags, then save the files with “Clear existing tags” enabled. This would properly remove the custom tags. Downside here is that it would also delete other custom tags you might want to keep. So you have to add those to the “preserve these tags” list in options.