This is something that honeslty shouldn't need to be discussed. Today I was at my 7th game on a flawless card, and one of my teammates straight up left the match 2 rounds in, leaving the other 2 of us to loose against a full 3-player team and our flawless cards.
(Freelance for reference)
Therefore, when someone leaves a flawless game intentionally, it shouldn't count against, this way no one looses a flawless due to a teammate leaving. Those who leave intentionally should get slapped with a long ban that extends each match they leave before completion.
Could this exact thing be abused? Maybe. But it being a freelance only thing could help lower abuses as you cannot see the cards of teamates, and helps you not have your runs ruined by someone who quit in a 1-2 match for no reason.
Edit: Updated
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I altered the post based on some comments. However, not that this isn't a perfect solution and I know it is, but it would be nice for the players who choose to play by the rules of the game. If people are gonna take advantage of them, they will no matter what. I see it as better to help those who choose to play the game correctly than to punish everyone because of mechanic abusers.