[GAMEPLAY] Shaco gromp bug, stuck at 10 damage bonus

I'm not sure exactly what triggers this bug, but I will describe what I do that reproduces this bug on several occasions.
The bug occurs when I play Shaco and whenever I smite gromp, throughout the game the gromp damage bonus is always at 10 magic damage every 3 seconds. It does not scale over levels/time.

I play Shaco and I build "Ranger's Trailblazer" then I proceed to build towards Tiamat. I can be level 14 and still have 10 magic damage buff from the gromp, despite smiting the gromp more than 5 times throughout the game. It doesn't scale over time for some reason.