I was Diamond 2. In one tranche (which updates after 5 wins or 15 losses) I had 3 wins, and on my 5th loss in Diamond 2 I randomly de-ranked after the game. It was not my 15th loss, it was my 5th loss–I only played 8 games in that tranche.
The system shot me from Diamond 2 down to Platinum 1. I was confused and annoyed, but I played another tranche, this time I went 5-4. The system de-ranked me from Platinum 1 to Platinum 3.
I played another 5 games, PERFECT 5 wins in a row. I carried most of these games, and my skill difference now is noticeable from the enemy team and my teammates. I have very few deaths compared to the other players in these games and higher stats.
It kept my rank at Platinum 3.
It feels like the ranking system randomly decided that I had lost 15 games in a row when I was Diamond 2, and it de-ranked me accordingly. The weight of those imaginary losses kept affecting my rank through several tranches now. It feels like unless I perform extremely well, I cannot reverse the weight of the imaginary losses that keep de-ranking me.
I promise that my skill level is equivalent to that of other Diamond players, I have no idea why or how it randomly de-ranked me.