I've never once had this happen to me.
People have always been able to pull off coordinated plays before, I don't understand why all of a sudden it's become an issue we need to blame Riot for.
Sure there ARE people who get unlucky- There's always one soloQ with every 4 man premade, but I don't for one second believe it's happening any more than what Riot say.
The worst I've ever had slanted against me was when I was in Duo with a friend and there were three in queue on the other team with the same Club tag.
Whenever I play with four teammates, I've always without exception been against another full team.
Maybe there's some huge difference between the way OCE and NA run their Dynamic calculations, indeed there would be more players to sort through on NA servers, though I'm sceptical at best on this subject after seeing the difference between playing against premades and playing against coordinated SoloQ.
I've played around a 50/50 split between solo and team games and the only time I was accused of being in a team was when I was alone-
But it was always because I coordinated a play or gank really well with someone else.
5 man dived bot lane because you're up 10 kills at 5 minutes doesn't mean they're premade.
It means you're attracting attention.
Coordination doesn't = premade.
Consider for a second in a game of SoloQ, you've got a 2 man premade on your team (top + mid) and so do red team. Their premade is jg + mid.
They see their botlane getting bent over and having a hard time, and decide to show up to help out.
From the blue team that might LOOK like a four man dive to get kills and indeed, sometimes they synergize really well, but that doesn't mean all four players involved are premade against you in your SoloQ game.
Just because it's possible, doesn't mean it's happening.
Just as because it's statistically improbable, doesn't mean it won't happen either.
I honestly think there's a bunch of people who figured out by typing into /all chat that they're "premade" that they have an easier time winning.