Its working as intended, Healing immunity cannot prevent the protection up that Enfys gains (stacking 40%). Yes, this is too powerful of a skill for a toon that counters with expose, and has stacking CC and CD (rest of encounter). Enfys can straight up kill toons who cannot one shot her (40k-ish health at g12+ and mods) and there are plenty of teams that she can destroy alone (zBastilla teams).
However, there are ways to defeat her (stun her and damage her after she takes a turn). Healining immunity does not prevent protection up but it prevents healing via health steal. Attack with Scav rey AFTER her protection up is down. Best way to manage Enfys is to chip on her protection while taking down other toons so that you can selectively attack and kill her. Saving her for last can be problematic.
She's a strong toon but beatable. My only issue with Enfys is that AI targets her unnecessarily or doesn't handle her well which means teams on defense is awful. I wish defense was great again.