I also experienced the healing. That, I think, must be a glitch because that makes no sense.
But again, this proves what Im saying.
1. You shouldn't be able to just sit and snipe enemies endlessly. That is a broken mechanic.
2. They shouldn't heal to full unless you take a long or short rest.
3. Once you do anything that's lets them know you're there they should be hunting for you.
4. Something I implemented in TT should be used in Bg3 as it is a standard tactic of phase spiders. They hide in the Ethereal plain and then pounce on you suddenly. So as you will see when I posted my next example of the matriarch fight, there should be no ability to sneak up on the matriarch. Phase spiders sneak up on you.