I have tagged some M4A files with Musicbrainz Picard. The Performer tag contains multiple values. Picard wrote all of the tags correctly, but foobar2000 only shows the last of the Performer values. I made a copy of one of the M4A files, wrote the Performer tag with foobar2000 and compared the files with AtomicParsley.
The only difference I can see is the case: "performer" vs "PERFORMER". Is this expected behaviour? (I should note that I'm using foobar2000 for Mac, latest preview version.)
Picard:
# AtomicParsley 01\ Overture_\ Lore.m4a -t
Atom "©nam" contains: Overture: Lore
Atom "©ART" contains: 古川毅
Atom "©wrt" contains: 古川毅
Atom "©alb" contains: 人喰いの大鷲トリコ
Atom "©gen" contains: Video Game Soundtrack
Atom "trkn" contains: 1 of 24
Atom "©day" contains: 2016
Atom "----" [com.apple.iTunes;performer] contains: London Symphony Orchestra
Atom "----" [com.apple.iTunes;performer] contains: Trinity Boys Choir
Atom "----" [com.apple.iTunes;performer] contains: London Voices
Atom "aART" contains: 古川毅
Atom "soaa" contains: Furukawa, Takeshi
Atom "soar" contains: Furukawa, Takeshi
Atom "soco" contains: Furukawa, Takeshi
Atom "soal" contains: Hito-kui no Oowashi Toriko
Atom "covr" contains: 1 piece of artwork
foobar2000:
# AtomicParsley 01\ Overture_\ Lore\ copy.m4a -t
Atom "trkn" contains: 1 of 24
Atom "©gen" contains: Video Game Soundtrack
Atom "©day" contains: 2016
Atom "©nam" contains: Overture: Lore
Atom "----" [com.apple.iTunes;PERFORMER] contains: London Symphony Orchestra
Atom "----" [com.apple.iTunes;PERFORMER] contains: Trinity Boys Choir
Atom "----" [com.apple.iTunes;PERFORMER] contains: London Voices
Atom "soco" contains: Furukawa, Takeshi
Atom "©wrt" contains: 古川毅
Atom "soar" contains: Furukawa, Takeshi
Atom "©ART" contains: 古川毅
Atom "soal" contains: Hito-kui no Oowashi Toriko
Atom "soaa" contains: Furukawa, Takeshi
Atom "aART" contains: 古川毅
Atom "©alb" contains: 人喰いの大鷲トリコ
Atom "covr" contains: 1 piece of artwork