From the mmr faq:
My interpretation is you can only lose a set amount of mmr per season, so it might take some people a few to get theirs where it should be.
They also said mmr matching is only used in legend and while playing with a star multiplier. Not sure they have implemented that right, I see a LOT of diamond 5 bracket players facing legends, the vast majority of players wont have a multiplier at diamond 5.