This is that the game does not think it has the potion effect or it does, but it is making the player invincible. This bug is kinda useful for making players invincible to damage, but there is another bug about this and if you are under the /effect command of Instant Health, where if you take too much damage from a kill, the game thinks you got killed by a command, but it wasn't that. I only discovered this on Xbox One, if I find it on other versions of Minecraft, I will post it. This was probably how C++ is coded. If Mojang can't fix it, consider us, Bedrock, lucky. Sorry if this is hard, Microsoft. This happens in all gamemodes.