Well, she wasn't born in Hell, and I don't think her soul was sold to Zariel (otherwise that would've definitely come up), only she herself physically. Devil blood also doesn't seem to curse tieflings to be reborn in Hell (again, would've likely been mentioned)?

I think the automatic rebirth in Hells only refers to pure devils and fiends and likely those who sold their soul to one.

And for all of Karlach's habits to run from her problems, I believe she'd consider this possibility in her adamant desire to get away from Zariel and Avernus... :"D

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