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the next time I talked to Shadowheart there was a conversation option to talk about how the Astral Prism switched from her to me

Its not a bug, the appropriate animations, dialogue, narrator comments dont exist. It can't be fixed. What had to be fixed is the player getting the artifact, though im not even sure when the artifact handover is supposed to happen naturally. Because they clearly wrote it with assumption that player already has it, it's not a bug.

Feel free to show me a scene where you dont have the option to hand over artifact if you dont have it, or to say "shadowheart has it" or whatever. I dont think those exist, so those arent bugs.

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hen you disguise every companion as Gith

No you dont need to disguise the party as gith, just the person initiating the conversation needs to be gith, that's all. The characters ignore what race your companions are. So all you need to do is just control someone like Lae'zel and youl get the dialogue you describe.