That is so funny.
Game Breaking Bug

But,,, did it take time for the rook to appear or the rook-capturer to disappear?
I think it is possible to move a piece from the board.
Who is the abuser?
I'll just take a look.

Oh... could you move the piece in the disappeared state?
Did you ask any of them if they witnessed it too?

geez, this reminds me of ChessNetwork having fun with the Blue Chicken Gambit on ChessCube:
https://www.youtube.com/watch?v=8KNaDDl66AQ
This bug occurs when a rook is captured, that belongs to a player that hasn't castled on that side, but would have been legally able to (i.e. their king and rook hadn't moved and the pieces in between have been moved). After the rook is captured, the player can still make the castling move, THOUGH THEY HAVE NO ROOK by moving king 2 spaces over and lo and behold, the piece that just captured the rook disappears off the board and the rook reappears in the position where in its "castled" position. Thus the player abusing this bug sudddenly has a 2 piece advantage by taking off the piece their captured their rook for free and having their rook reappear on the board. Can chess.com fix all the castling issues already? I witnessed someone doing this in Teams btw, but likely the same issue is present in FFA.