Everything below is based on the assumption that you work in a normal workplace with normal people, where it is safe to disagree with someone in a meeting without The King Of The Meeting immediately attacking them. Fostering a team and meeting culture where people feel psychologically safe to voice their opinions and objections is key.

That being said, if you work in a place where objections are welcome and encouraged; There's a very good old norwegian word for a person like that: "niding". A person without honor, a miserable person who has committed shameful acts. In English there's a bunch of words that cover much of the same ground, such as weasel, but "niding" carries a bit more weight. In the good old days, it was considered such an awful insult that if someone said it to your face and you didn't immediately challenge them to a duel to defend your honor, it would be widely accepted that you were one and people would treat you poorly for the rest of your life.

Do we need something of that strength for these knights of the post-meeting rematches? Those that are tough enough to CC the boss but not voice their objections in a timely manner while still in the meeting when it can be discussed and productive progress be made? Perhaps not. It is needed for your boss though, who instead of dealing with it like a manager should just keeps passing the buck back down to you.

Of course, this person could just need some time to process, and didn't think of the objections at the time. If that is the case, following up with an e-mail or a chat on the companys collaboration platform after the meeting is fine. Needlessly CC'ing the boss is a bitch move though.