Flex 5v5: Placed High, Being Matched Low?

Just wondering if this is a "normal" experience for a lot of people? During my provisional games I progressively got matched with and against higher and higher ranks and was eventually placed in Plat I for Ranked 5v5 Flex. During the provisional games, I was mostly being placed with players that were between Diamond IV and Plat II.

Suddenly, after being placed at Plat I, my first game out of provisional matches put me with and against players that were between Gold II and Plat V.

This is the first time I've experienced such a dramatic drop in skill between provisional matches and regular matches. It's a little discouraging to place high and then keep dropping down rather than climbing up, as I would imagine the general design of ranked is.

Basically, I want to know why, if my MMR isn't that great, I was placed that high to begin with if I'm only going to get matched much lower? Has anyone else experienced this? I'm not saying I am perfect in every one of these matches, but it's getting frustrating to win lane but lose the game due to teammates that are at a much lower skillcap.