"I Did Good in Promos But Got Bronze WTF !!!1!"

Your MMR gets a soft reset.

Wherever you got placed last season contributes to where you get placed at the start of the next season.

If you got placed Bronze 3 last season, getting put in Bronze 5 is not unusual even if you won most of your games. The point of this is so you have to climb. It's not going to put you higher than where you were or at where you were.

If you placed Bronze or Silver last season, and got placed Bronze now, its not unusual, the system is not broken, I assure you it is normal. Just climb back up to where you're supposed to be.