Pretty much everyone.

I remember someone posted a video of something like this happening, where they won a MM game and completely deranked to level 1. I'll see if I can find the video of it or if someone will post it before I can but I've seen this bug before

part of this happens because the MM screen won't display progress if you leave before the game shows the final screen. It'll queue up all your elo changes, sum them, and display them all at once the next time you finish a map, so if you lose two games, leave as it ends, and finish a game you win, you'll lose one game of Elo