It's actually because Minthara is coded and build as a party member. I believe it happens with Karlach as well, though I'm not 100% certain about that.
Yes, it's exactly that as far as I know. BG3 wiki also confirms her status as a future companion.
Even if you think about it:
- She is mentioned as an ally and lover if you side with her, by the Narrator in ending cutscene for Act 1.
- Just like PC and Companions, she has a tadpole.
- She has a romance scene, just like the Companions.
- I remember reading about game files referencing a drow companion, I can find this for you if you want, it was being talked about on steam forums.
- There is a good story reason for her to appear later. She even promises that we will meet at one point again. Moonrise Towers seem to be the best moment for this.
- Since she doesn't know about having a tadpole, this could mean a possible quest where we turn against the Absolute with her and that could lead to some kind of a redemption arc.
A little bit off topic, but
Halsin looks like an excellent candidate for a companion too.
Would be great to be able to choose them as a companions even if we side with the opposing faction. (Thanks to persuasion/surrendering/capturing/whatever)
