I agree. For my playthrough where she and Wyll did go to Avernus, I barely had to encourage her at all at the end. I think I selected the most neutral option available, but since Wyll had already told her (unprompted by me) that he would go with her, she agreed. It doesn't make sense that they behave so differently if the player is gone, especially if it's spawn Astarion who doesn't have the choice to stay.
