To be honest with you, I really don't know if that's either a bug or if it's really what the system now may say about your in-game skills. Also, a friend of mine got that problem as well so yea, I think that it probably appens to a lot of people in the lower platinum ranks.
The problem you got with your rank is not as big as mine by far, I was silver III last season (I never played that much of ranked games)
This season I duoqueued with a friend of mine. On the way though, we found in your placements people that were gold last season, etc. He's ended his placements, carried with me all the games (except two that I didn't play because I ward afraid this could happen) and got to bronze V. It looks like something really funny though, because he has 1500 normal games wins and he was in bronze after deranking because he wanted to. Now he's there and he doesn't want it anymore, but either ways, bronze V is the best rank at the momment, so stay happy that this didn't appen you. I am most likely going to follow his path, but I don't think you can do much about it.
Good luck on your next games!