There are a lot of things that can affect SR change besides performance SR. You are correct that performance SR does not exist in diamond+. See How Competitive Matchmaking and Rating Works (Season 20) for what it might have been.
Regarding gaining 33 in diamond:
Perhaps you left the previous match and got a 50 SR penalty, and now have an SR buff.
Perhaps you have not played on this account/role in a while and have a high MMR/SR uncertaintly.
Perhaps the match was unfair, and you were expected to lose.