I always kill Gut, free Halsin and then move onto Dror and Minthara. In my latest FH playthrough I did the exact same as any other time.

The bug occured when I did things in this order:
-Kill Gut
-Free Halsin
-KO Minthara
-Long Rest
-Kill Dror

I tested the exact same thing in another run but this time I did NOT long rest between KO'ing Minthara and killing Dror and the quest completed as normal. There is something weird that happens to the game when you long rest after KO'ing Minthara.