Spoilers for *THE* major turning point in Shadowheart's story
I was with Shadowheart and Nightsong, and I failed my persuasion roll to get her to spare Nightsong. When that happened, I told her that I would fight her before I let her kill the Nightsong. She flew in to a rage and was ready to throw down with me, and then suddenly I was in a cut scene with Nightsong. I freed Nightsong, and she did her thing, and then the cut scene was over, and I was back in Shadowfell, and Shadowheart was just gone.
There was no fight with her, no final dialogue, she just disappeared during the Nightsong cut scene.
Is that how it's meant to play out, or did it bug out on me?
Yeah, seems like it bugged. I had to do the same, and we fought and he body was there. There's no cutscene or anything when you kill a companion, but they shouldnt just disappear.