What the hell happened???
This is what happened when I was feeding my pet during testing
SpoopyDuck
#1
AKLDragon
#3
I would guess just a visual bug, probably report it in the Trivial Issues thread, or maybe mods can reopen the Testing bug Megathread, or you can post it in the current Public Testing thread.
Edit: @moderators it would probably be good to reopen that Testing Bugs Megathread.
Seelpit
#4
Seems like a bug that might occur when you feed your pet the exact amount of feed they needed for a level up. Just speculating here, nothing concrete.