Once mobs and players start doing lots of damage against someone with a lower damage multiplier, no knockback is taken. Invulnerability frames do not appear either, as someone doing high enough damage can kill the player in moments.

Steps to reproduce:

1. Add the damage sensor component into the player.json with a tiny damage multiplier, something along the lines of this:

   "minecraft:damage_sensor": {
      "triggers": {
         "cause": "all",
         "damage_multiplier": 0.01
         }
      }
   

1. Open a test world and spawn a hostile mob (or invite another player to join), we'll refer to them as Attacker
2. Use the effect command to grant Attacker a notably high strength level (something like strength 5 should be good enough, although I did this with an enderman, so it may need to be higher for weaker mobs)
3. Go into survival mode and allow yourself to be hit by Attacker

Expected Results:
You should take a damage tick knockback, regardless if the attack ultimately hurts you or not.

If Attacker is a player, damage immunity frames should also prevent Attacker from rapidly hurting the opponent.

Results shown:
There is no knockback taken. Apparently at some specific level, proper damage application knockback stops being accounted for.

Update 1: I have created a new world specifically to show off the bug in great detail (in the attachment below). This world has a behavior pack where the player.json has the damage sensor modified with the damage multiplier. In the case that the bug must be recreated from scratch, however, I have already explained here how to recreate the issue.

Update 2: This also affects the breaking of armor. Going based off of Golden Helmet's comment as of the 12th of March, 2023, receiving fatal damage being calced beforehand also prevents the durability breaking of armor.