Hi, this is Black Survival. 
The issue that appeared after the December 22nd Patch, and allowed players to be demoted when playing Pre-promotion matches has been fixed through a small patch today at 5:30 (GMT+9)
1,000 Gems will be distributed as compensation to players who have been demoted to an inferior Tier when playing Pre-promotion matches.
We apologize for the inconvenience you might have encountered.