This is not a bug. Your pet is leveling exactly how it was meant to in the official. Killing snowman does not guarentee instant level. It gives your pet the maxium experience (15xp per kill

).
Sometime ago a member of the community changed the pet experience to 1 so that you only required 1xp to level. My guess is that they stopped at 110 because the game isnt meant to go much higher than that so they saw no point.
Now to change the pet experience. Open up your elements.data in the an elements editor (sEledit). Browse the lists until you find pet experience. It should be labeled so not too hard to find. Once you find the list you see a list of levels and pet experience. Scroll down to levels 110+ and change the experiences from ? to 1. Save your elements and export. Log in and you are done your pet should level with 1 kill again.
I found the pets to be utterly useless at higher levels. I only leveled to 109 but even then I found that the pets could easily be killed by other players in pk. For this reason I see no point in leveling them or you higher. Also you become way to strong for any of the mobs or bosses in the game.