Ghost monsters in Hell take no damage despite no immunities

Ghost monsters in Hell take no damage despite no immunities. I have to teleport back to town and back again to remove this bug. Only when I do that they start taking damage.

I play Necro and use Bone Spirit. The Ghosts do not have immunity to magic damage, so they should be taking damage from the Bone Spirit but for some reason they don’t unless I travel to town and back again.