When giving yourself high levels off the resistance effect in a singleplayer world, for example level 255, the effect works just fine, making you take no damage from most sources.
The effect is also shown in the inventory GUI.
If you now leave the game and then rejoin it, the effect is still shown in your inventory GUI, but no dammage is reduced.
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Arisa Bot
Thank you for your report!
We're tracking this issue in MC-10755, so this ticket is being resolved and linked as a duplicate.
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Unsupported amplifier: MC-10755.
BTW, level 5 already makes you take no damage. It's 20% reduction per level.