TCEC, probably the most prominent long-form chess engine tournament, does exactly that. It starts them in a pre-played opening position, usually ~10 moves or so

TCEC, probably the most prominent long-form chess engine tournament, does exactly that. It starts them in a pre-played opening position, usually ~10 moves or so into the game. There are still a lot of ties, but fewer than if you let them play from neutral. Each engine plays both sides of the position (white and black) so it's always a bit of a cool upset when an engine wins both the white and black side of the same starting position.