Why does my purified Pokémon need to be healed?

I was purifying a few of my Shadow Pokémon when I suddenly noticed the Zubat I had purified apparently had taken 1 damage somewhere. I had literally just purified it, I was still in the Pokemon screen; I didn't use it in battle or anything.

I also noticed many other purified Pokémon (Rattata, Bulbasaur, Golbat) with very little damage, but still hurt. I tried purifying a fainted (0 HP) Shadow Squirtle, and it came out at full HP. I can't trade them if they're damaged, and I don't have infinite potions to heal.

Where is the random damage coming from?

Solution 1:

This is a bug that's been in the game since day one actually. You see what happens is the Pokemon's level is increased to 25 (or higher), given full health like in evolution, but then its IVs are increased. Usually +2 Stamina IVs will be enough to increase HP by 1, the game doesn't heal this health so it will be 1-2 health down.