hmm.
Killer Pawn Achievement

Received this award even though I delivered mate with a bishop. The system is very buggy.
You don't have the killer pawn achievement or killer bishop. You received killer rook from a game yesterday.

For the purposes of these awards, whatever piece you moved last "delivers" checkmate, even if it's mate by some other, discovered attacker.

It appears that if you checkmate with a pawn promoted to a queen on the turn of promotion, it will register as a pawn checkmate.
I also received the achievement because someone resigned after I did a pawn move.

I legitimately got it here: https://www.chess.com/live/game/66594849303
It appears that if you checkmate with a pawn promoted to a queen on the turn of promotion, it will register as a pawn checkmate.