There is no rule that says it is illegal to break the huddle with 12. The substituted player must leave the field immediately. In this case it appears to me that the player coming on the field stopped and looked back to see what the play was that was being signalled in. That player and the QB got the play at the same time. At that point in time the player being replaced understood he was being substituted for and left the field. It appears to me that there was no attempt to deceive, the substitution did not violate any substitution rule, and there was no illegal formation.
It does appear that the defense could be confused prior to the RFP as to who would be playing for A but there are many substitution situations which we all agree are legal where B doesn't know exactly which 11 players are in the game until after the RFP.
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