Whenever regenerating health or obtaining health through the instant health effect, players and mobs get invincibility frames, being unable to take damage unless the damage is high enough that it one-shots them.

While it is noticeable with lower levels of regeneration, it is undeniably noticeable with higher regeneration levels such as regeneration 4 and up, and most definitely noticeable with the instant health effect, being able to ignore otherwise serious damage while in the brief process of healing.

Steps to Reproduce:
1. Obtain an instant health or a regeneration effect, preferably regeneration levels 4 and up
2. allow yourself to take damage, be it from mobs, fire, etc, so long as the damage isn't enough to instantly deplete your health
Observed Results:
No knockback or indication of damage is taken at all, you are practically invulnerable.

Expected Results:
The entity receives damage and takes knockback, albeit it will recover from the damage fairly quickly.