Don't know if it is related but I just had three fights with wraiths and shadows and one of them kept going running in, hitting then going invisible. Neither Faerie Fire or See Invisibility did anything. For some reason (WIS?) Shadowheart was the only one who made a save which made her able to spot the thing.
This was in Act 2 - around the Mason's Guild and the building before that. It wasn't life-threatening it just made the simple fights drag out far longer than needed.
I've used See Invisibility on the hag before and it worked though I'm not sure it was this playthrough.
