Yeah, it's really broken. I can't help but feel the way it seems to handle duos is a large part of the issue, though.
See, I've friends. Some of them are way better than me. Some of them are way worse. Like way, way, worse. We're talking some of them are actually Bronze 5 worse. As for myself, I've never played ranked, but based on the people I see when I solo queue, I'd guess I'm about average. Mid gold range, maybe.
Anyway, by way of example, sometimes I queue with my way worse friends, because sure, they may be really godawful at league, but they're still my friends. I'll lose a lot of those games. It's fine. I expect to lose. We get the weirdest, most lopsided matches when we queue together. And we all laugh about it.
But this seems to absolutely murder my mmr. The next time I queue solo, I'm suddenly surrounded by, like, the worst players I've seen ever. And I'm a god among men. My lane is a total stomp in my favor. It's simply unfair.
The system really ought to adjust your rating only minimally if you're in a match with vastly differing mmrs, because those matches are just never balanced. That way, when you return to queuing solo, you'll pretty much be right where you left off.