I've recently come across this scene while checking vids on YT. All comments said this happens when she stays loyal to Vlaakith.

If you want to get a happy ending with her, she has to be shown Vlaakith's mind during her visit in act 3 (after succeeding an arcana check), be persuaded to spurn her, or not be stopped if she decided to denounce her on her own after witnessing the events at the créche and allying with Voss.