I did the test I mentioned. I used the admin panel to get two fresh thralls, and fed them different foods. For both, changing which attribute growth chances get increased was possible, but it required exiting and restarting the game. But cooked pork rinds worked different. When I fed them that, the bonus changed immediately, without needing to restart the game, and then became stuck. I suspect that food is bugged. Not only is the 25% damage boost impossible to overwrite with other foods, but the growth chance increases to strength and grit cannot be overwritten either - and unlike the damage boost, they persist after restarting, until the next level. Feed that to a level 20 thrall, and it is permanent.

Also, thralls being able to get food from the thrall pot when much too far away is another bug. But that is probably what happened. I have previously seen a thrall suddenly with cooked fish in their inventory, which I had not given them, but I had a stack of it in the thrall pot.

Apparently, the only viable workaround for this issue is to never have any food in your thrall pot except the one you want to use for leveling attributes.

By the way, there is one food for human thralls that also does not affect their attributes - roasted haunch. Feeding that to a thrall who already has a boost to attribute growth does not remove it. So, for human thralls, cooked pork rinds cannot be overwritten, and roasted haunch cannot overwrite, which is just plain weird.