Consequence of the following.
19.5.2 A fielder who is not in contact with the ground is considered to be grounded beyond the boundary if his/her final contact with the ground, before his/her first contact with the ball after it has been delivered by the bowler, was not entirely within the boundary.
So Carse was deemed “grounded beyond the boundary” on his first contact with the ball, so six. Brook wasn’t on his first contact. Going out of play after the first contact is OK unless you are actually grounded and touching the ball at the same time. Curious law. But correctly applied. Good umpiring.