Why the replacement???

I usually play flex with a group, placed mid gold this past season and the flex placements put me at Silver 1, kinda sucked but I figured it wasn't too bad, just played a single game alone to warm up and got placed in Silver 4 instead...

What the fuck is that? My MMR changed an entire rank from 1 game, that's one shitty stab in the back from Riot, especially since there wasn't really a reason for replacing other than another "we fucked up, sorry".