Monsters becoming "Invulnerable" halfway through a fight
I have noticed this too, it tends to happen most when the Z position of the mob is greater than the player (closer to the surface with water level being Z=0)
^ that.
It’s with the position of the player or your pet/minions (if you have one) relative to the mob. To you it looks close enough but to the programming, it isn’t.
The workaround is to stay really close or find someone to help you (or do it when the area is crowded – there’s no KS concept anyway, so tag everything you can when the area is crowded, or help everyone you want).