When a Player (or Mob, I don't know) with Resistance 5 or more (which makes the Mob almost invulnerable, Exceptions e.g. /kill) get "hurt", they appear once red blinking, in the 1st Person the Screen "shakes" and a hurt sound appears.
Tested with Fall Damage.
May be only on Fall Damage. (I don not know.)
Related issues
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kumasasa
Effects above level 4 are not supported, see MC-10755
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Also, you can still be hit without taking damage.
I*m going to try to figure out the unknown things at weekend.