When a rat has a single food item in its hand and eats it, it heals by 1 HP instead of the hunger value of the food. I believe this is because the food item check happens after the food is consumed (specifically this.getHeldItem(Hand.MAIN_HAND).shrink(1);), which is why it only occurs when there's only one item left.

I have not tested this bug in other versions, but I believe it still exists in the 1.16 versions due to the rat's eating/healing code being the same in those versions.

was fixed in PR linked above