Tried what you suggested. I fed Ansina feral flesh, followed by shredded roast. Her stat bonus did not switch to grit.
I had a theory about the thrall pot. It occurs to me that even though she was following me in the Unnamed City, her last guarding spot was within range of the thrall pot. It makes no sense that she would be able to eat from it, or that she would choose that particular food, but that could be the cause of the bug.
It also occurs to me that the strength and grit bonus comes from cooked pork rinds, which is the only food that gives a 25% damage boost, which takes precedence over other foods. If that also applies to the level up bonus, that could be why it is stuck, because it overrides all other foods.
I’ll make a new thrall and test if I can get that one stuck as well.
There is something suspicious about the whole level up system. Why is it that the first and only the first item you feed to a pet each level matters, but the same does not apply to human thralls? The system should be the same for both. I also wonder whether those growth chances are somehow rigged. I have a single horse, which had a 100% chance of growth to vitality, and every level I fed it vines and it got two points. Odds of that should be 1 in 76296, and I don’t think I’m just that lucky.