I started a new PT a while ago (dark urge) and Karlach's romance seems to be triggering again. (As stated above, it didn't work in 2 previous origin PT"s).
I'm still in act 1, but we had the "pillow talk" at the ttiefling pary, declaration that we want to be with eachother, and the sparkling kiss. And Karlach's permission to go with "the angry alien" until her situation is fixed.
So maybe that last small fix did something, or a dark urge is not subject to the bug ?
