I finnished s1 last act and was matched against plat/low dia this act for some reason, won 1/5 placement games and got plat 2. what the fuk, can somebody explain?

Hi everyone, I've been playing since the beta. I'm lvl 88 on Valorant and just reached Immortal for the first time this act. I've played thousands of hours 1.6 and came back to FPS something like 1.5 year ago with a completely washed up...