I just had the same happen. Won an evenly matched flex game (so the MMR says and based on flex leagues) and got +11. Saw a bunch of these posts recently too and didnt look into it since I thought it was some rare thing but had it happen to me too
Red Post:
{quoted}
LP gains and losses are based off how likely you were to win that match. The system expected you to win, so it gave you a normal amount of LP. The match where you gained 21 LP was a match that the system thought you might lose, and since you did end up winning it gave you more than usual!
from https://boards.na.leagueoflegends.com/en/c/help-support/nQhlxWW0-why-did-i-gain-11-lp
but that seems super fucked up. My flex games I was getting +22 and up to +28 at one point meaning it's trying to get me to my proper ELO. Have a winrate over 50%, solo is in gold, and flex was placed bronze 1 then moving up through silver 4 now. So if it expects me to win, doesnt that mean matchmaking is piss poor when it cant give an even game? If the person who is crushing shouldnt be in that league, shouldnt they be moving up FASTER rather then SLOWER?