Why did I lose 25 elo in a match I've won?

I lost 25 elo in match I’ve won and don’t know why. The friend I queued up with received 25 elo for the win. I heard about this happening because someone in my team might have gotten banned in the meantime. However if this is the case, does it also apply when I didn’t queue up with them? Because I’m not aware of anyone I queued with that got banned and if it was just a random guy, it’s very unfair to me to deduct the elo from myself.