This happened in my current playthrough, I did the Gauntlet without her, she showed up and complained I robbed her of making a decision, then left for good. It gets weirder in Act III, you show up in a certain area and are told to bring her with you, but you can't because she has disappeared. I was hoping this would be expanded on more, but nah, she just left, didn't even return to her cloister. Maybe they could've had her be one of the captives at Moonrise or in jail in Baldurs Gate, but I haven't seen that as of yet.

On the artifact, I had a weird issue with that also. It apparently
comes to you of it's own accord
, but this never is shown in any cutscene or explained. It's mentioned, and she tells you
to just keep it
but nothing beyond that.