Example: I'm Gold I, and my LP gains are low as hell, I get matched with Diamond players (Low MMR huh?) and if I win, I get ~ +5 LP.
I then proceed to lose one game while said Diamond players are on my team and I lose a whopping 26 LP.
So Diamond players get placed on your team, you most likely have a higher MMR as a whole than the other team and thus a higher chance to win against the other team. You still don't win so you lose more points because the game is thinking you should have (hopefully makes complete sense). Same scenario and you win. The game was already expecting you to win because you had a higher MMR than the other team, so you don't gain very much. It really doesn't seem that complicated.
If the pair up is 10,000 MMR verse 6,000 MMR (obviously super fake numbers) the game expects the 10,000 MMR team to win and thus they should be punished more heavily for losing, and rewarded less for winning. That's what it seems like your example is and I'm not sure how that doesn't make complete sense.
[e] A problem with the system would be if it didn't reward the winning team that beat your diamond team larger (ie. a larger LP gain because they shouldn't have won). Do you know if it did or didn't?