What's wrong with the Flex queue system?

My account's name is Puddin1mida in the NA server. I'm now plat 2 in the flex rank, and gold 3 in the solo/duo rank. I don't have any high MMR. HOWEVER, I was always queued with Diamond 1s and so on. The most ridiculous thing is that I just finished a flex game with two challengers on the other team, and the rest of them were all diamond 1s, while my teammates were only diamond 4s and me plat 2. This is definitely not right and severely affected my gaming experience. I believe the MMR matching system went wrong somewhere since the rank game was designed to put similar level of ability players to compete, instead of letting those top players to ruin our game. I need a response and an explanation from Riot. This is not fair!