Pvp victory but loose points

I had this a couple times. Most recent was 2 mins ago. I win the match by killing their last monster, it says Victory. Then I get back to pvp screen and it says: -31 points, and it doesnt add a victory…

My first time, hearing this… Most of the time you lose when winning in the middle of the game but this I never experienced before

Occurred twice during this pvp.