I believe that this is an actual bug. I deranked from Silver 3 to Bronze 4 in support, I peaked plat and stayed gold most of the season so theres no way my MMR was that low.
Then I won 4 games lost 1 and went no where - all stats high, best on the team for heals and assists, low KD
You are not alone in experiencing this bug. Here’s a link to my post in case you would like to read more about this issue: Comprehensive Thread Of All Top 500/Competitive Bugs. I’ve compiled a list of posts regarding similar bugs under: Incorrect Rank Adjustment. Please help in spreading the word about this megathread and other related threads, we must demand the Developers to give us the rewards we rightfully earned.