Ive got a pet sabre with a 52% growth chance in vitality, feeding it savoury meat gives +14% growth chance, giving it overall 66% growth chance.
Disclaimer, i am aware you feed thralls/pets each time they level up and until you see the +‘X’ in brackets next to their growth.
I have levelled this thing up to level 11. And it has only levelled up it’s vitality once, and that was because I took the meat out after realising it was not levelling up it’s vitality at all with 66% growth. Yes theres a 34% chance it wont, but probability suggests otherwise. And it is interesting that it’s vitality levelled once i removed the food that gives it +14% growth.
This update has messed with alot of things. Please take a look at this issue. This has also happened on other thralls and pets too.
Could be that, could be just bad luck with RNG.
Had one of those ridiculous RNG freak-outs just recently when i decided to level up a wight in my single-player game. Just check out what it was supposed to be leveling up (it was fed +Vitality food the whole time) and what it actually did.
Honestly just looks like bad luck to me.
66% chance isn’t high. On Average you would get the increase 2 out of 3 times, but the sample size (here 11 levels) is not significant enough for reality to align on probabilities.
If you had to level up a follower for 3000 levels then yes you would have a result closer to this 2/3 times, with your follower getting vitality close to 2000 times out of the 3000; but with only 11 iterations, it’s unreliable and just bad luck. And yes it’s dumb and unlucky, but it’s just RNG being uncool 
The odds of leveling up 11 times and getting 1 vitality at 66% is approximately 1 out of 6666. Quite low, but still well within realistic bad luck.
Get rid of that freaking thing, @Chrysa1is ! It will bring bad mojo to your base! 
Theres a bug with pets. They for some reason are passively producing meat and honey in their inventories… just checked my rhino and it had 2 stacks of honey, pork, savoury, feral and exquisite meat in its inventories. Id argue its tampering and bugging our growth chances.
Is there a thrall pot nearby?
Animals like to steal from it, no matter what food is in there. Hares munching on human flesh? No problem!
Ah yeah there is. Didn’t think that a rhino would take meat. Fair enough! Haha
