Shouldn't matter I think. You didn't have her kill Nightsong, did you?
Either way there were bugs with the scene not happening but the game assuming it had. It tends to happen a couple long rests into act 3 already, assuming you went the 'good Shadowheart' route. Could probably be the first one but it seems some other events are queued up with higher priority (e.g. Vlaakith showing her big ugly face). So if you have a save from the start of act 3, just spam long rest and see if it happens now. Note that you have to talk to her before it happens (the usual ! mark at night time).
