Edit: someone answered this question on reddit. The issue was, I was doing too much damage and she was bugging out.
Lillith is bugged for me. I tried different characters, different torment levels, singleplayer and multiplayer. Every single time I get her life bar just below half. Then, no matter how long I keep hitting her, her life bar will not reduce by a single millimeter. I see other people play Lillith in the party finder. So I presume other people can still kill her. I’m worried this will keep happening when the next season starts. Is there a fix for this?
If what I said doesn’t make sense, I made a clip of what happens during the fight and attached it to the same question I asked on reddit.