Hi I've been leveling up quite a lot of pets, and I never came across this issue. I have a Brainy Assistant pet, which has a pedigree of 95 and I was excited to see it's talents and of course this week has the double pet exp, so I took advantage and fed all the high level snacks I had, but I ran out of energy before I could level it up.
My friend suggested to buy the Pet Crumb pack and feed those to it to get easy experience.
I started doing that and by the time I looked at my pet's experience bar, it went OVER the requirements for reaching Ancient, but it never leveled up. I even went to log in and log out, waited for my energy to come back and play a game to feed it. Nothing worked it just continues to increase the experience bar but not actually leveling up to Ancient.
Right now it's experience bar is 585/525 (525 being the amount of experience that one needs to go to Ancient level.)
I don't know how to fix it, I looked around for hours for answers and I really don't want to label this pet as a failed pet either.