There is about 2-3 seconds (sometimes longer, sometimes shorter) before a match-up is locked in, and that gives players a chance to leave, especially if they feel like the matchup is not in their favor. After that you can’t leave matchmaking without playing out the game.
This resets the whole matchmaking process for everyone.
When I sit in queue for 10+ minutes and I get a matchup that is reset because someone leaves, that really sucks.
At least for PC, “[E] Leave Matchmaking” needs to be disabled as soon as the opposing team is shown.