When riding an entity, when you take damage you are able to take damage again 2x the normal speed. This works through potions, player damage, mob damage, and any other type of damage that can be distributed on the player or entity who is the rider. I'm not sure what causes this bug, but it has been around for a long time and hasn't been fixed. Since a lot of things in 1.13 are being optimized, this would be a great addition.