Demoted to Bronze 2 after winning in Gold 5 (Ranked Flex)

Hi First of all , i'm not sure if this is really a bug , or maybe i just didn't get the rules of the ranked flex .

So this is what heppened :

At the 12th of november I finished my placement's games, finished 8/2 and placed in Gold 5:

http://imgur.com/a/afkko

Today, i played in a ranked game ( Got 97LP before the game ) , won it , and this is what it showed me :

http://imgur.com/a/mSJtQ

I'm not sure if this is a bug or not , but i'd like to get an explain for what heppened here . Thanks.