Lots of people including diamond haven't really put any serious time into playing flex, so their flex mmr isn't quite representitive of their skill levels. this can lead to some stompy matches, which occasionally brings lower skilled people up to higher MMR than they should have, and it just leaves the whole system pretty messy. Since the system is a bit messy, people then don't bother to play more games and get their MMRs to settle around their skill level, and the problem persists.
Over a few seasons if people actually played it somewhat seriously matches would probably get fairer (although with premades being kinda the point, theres always gonna be much more skill variance compared to solo/duo).
I actually don't think it was that bad early on when everyone just had their starting seed MMR which was based on their old dynamic queue MMR from last season, but dynamic queue also had lots of people at MMRs they didn't deserve, which means their flex mmrs were wrong too. Then again, I am somewhat biased since I managed to place plat 1 in flex by some miracle, so probably skipped a lot of pain.