Ranked Flex just got fixed after they “crossed some wires with matchmaking”, according to Riot Socrate’s post.
He said that the problem started when some players’ MMR was incorrectly set to 0 while some high-skill players lost a lot of MMR. From the other side of the spectrum, low-skill players inexplicably gained MMR further worsening the quality of matches. Tiers and divisions were also recalculated to incorrect values therefore placing some players way below their skill level. There were some reports of Diamond players being placed in Bronze after their placement matches.
This problem affected everyone, including our own server. Despite Riot fixing the problem on Nov 14 according to Riot Socrates’ post, Riot Spiffy only posted about the problem that day. And it was only yesterday that the update went live in our servers.
With Riot introducing two (2) separate ladders for this season (Ranked Flex and Ranked Solo/Duo), this problem will hopefully be negligible by the start of the Season 7. After all, the preseason is the time for some crazy (albeit some were unintended) changes.