mp3tagの更新履歴で
[2017-02-06] NEW: added support for iTunes-specific ID3v2 grouping tag field GRP1 as GROUPING.
[2017-02-05] NEW: added support for reading ID3v2.2 iTunes tag fields for classical music MOVEMENT, MOVEMENTTOTAL, MOVEMENTNAME.
この辺の関係かもしれない、GRP1は確認してないけど