Good to know! I honestly forgot about that. I'll probably wait until after the jam is concluded just in case but good to know for the future.
That...is really odd. Outside of the brief duration when they flash white (and admittedly a bit longer for the Necromancer's skeleton minion -- just long enough for the hurt animation to play) they shouldn't be invincible ever. Do you know what attack they were using / you were using if you don't mind me asking?