I’ve tried to fight Lilith multiple times now and she just stops taking any damage right around half health during her first phase, seems to start after she spawns her minions
I can confirm, did her today and each time i could not damage her after 60-70 % of hp in phase 1
That is part of the mechanics of the fight, you have to survive the first phase before moving into second phase. Then survive the platform phases before you can kill her.