I'm not sure whether or not this is a bug.
Observed behaviour: in my latest run, Brielbara was missing, so she couldn't give me her quest. (Although it turned out that Yago was still there in the Low Lantern, so I got his Book of Curses, but obviously there was no use for it without Brielbara.)
Expected behaviour: Brielbara ought to have been outside the Splurging Sturgeon waiting to give me her quest.
Caveat: I don't recall Brielbara failing to appear in any previous playthrough, but I'm not dead certain. Is there, perhaps, some condition or trigger governing whether or not she should appear? Possibly this time there's some intended reason why she wasn't there, but if so, then that's news to me. Can someone clarify whether she should appear every time or only under certain conditions, please? (So far as I know, she ought to be there every time, but maybe I'm wrong about that.)