Big level problem

[Niukill]

I'm sorry but there is a big bug in the game. The pokemon go down from lvl after going up like a while ago I finished off one of a gym with my Kecleon lvl 37 and it went up to 42. When I did the fight my Kecleon went back to lvl 37 and this has been happening since Hoenn. It's quite annoying and frustrating due to the hours I spend leveling up and in the end they are useless and I can't talk about the story. This problem has been happening to me for quite some time. Please fix the problem as soon as possible.