User blog comment:B214/Question and Answers/@comment-24098614-20131106231935/@comment-1359322-20131110004242

The "bug in the system" you mentioned is on the Sacred Gear system not the Evil Pieces system. Mutation Pieces aren't bug in the system, they are part of the hidden secrets in the Evil Pieces by Ajuka Beelzebub, Issei Pawn Pieces becoming Mutation Pieces is part of the secrets. Also the Evil Pieces becoming stronger with the master is another of the secret.