I have my HP alert set to 60% - and I use Ginger Ale, which restores 75% HP and MP. I'm training at Ulu City Center, and killing Petrifighters. One melee hit is around 2800 damage, and takes me down to around 1000 HP. After two hits, I'm brought down to around 450 HP, which is under 60% of 1572 HP (628), and my pet should use a potion. This happens most of the time, but not always. When it doesn't happen, and I don't use a potion myself, I die. I don't know why it happens, but I have two theories. My first is that the extra calculation that happens with Magic Guard causes auto HP to sometimes fail. My second is that pets can not loot mesos or items at the same time as they use a potion. Edit: The bug always happens when you use Teleport into an enemy and take damage, probably since you're in the skill animation. ⠀
If your timing is unlucky your pet's auto pot won't work properly. For example if you get hit and use your Blizzard at the same time. Ikr it's annoying but you get used to the timing once you know about it.