I actually have Way way more over 1m

I believe health is stored as a 16 bit integer so it cannot go over 65536, if you set your health to more than that then it would get capped at 65536. If there is another mod that changes how health is stored to a 32 bit integer or something (though I've never heard of a mod that would do this) then RPG HUD would need to add compatibility with it.