Placement and roll out potential fix question

This is what happend to me.

Im Plat v 80LP in Solo/duo 2016 7w/3l gold 2 in flex. The fix for matchmaking issue in ranked flex happend, "you will need to play one additional placement game to determine your new ranking." my stats was 12w/6l (+5w/+3l).. I managed to get to Gold 1 promo. before this "fix"

Then got placed in Silver 1? Please tell me why i should fall 5 division down in 1 game...