I'm on a custom 1.16.5 pack, and mobs from Alex's are doing damage in a way I can only assume is a bug. Dropbears are doing 2 hearts of damage about once or twice a second whenever they have you in line of sight. This ignores armor and resistance, and happens at any range.

Similar thing with the bone serpent, but the damage is even faster. I go through a nether portal, and I'm dead in literally 2 seconds from a bone serpent I cannot see and don't believe I have LOS to.

Oh and a crocodile just teleported me into its mouth from 30 blocks away. something seems seriously wrong with whatever range/line of sight math is being used.