I tried a bunch of things, talking to him with Sebilel or Ifan while the other was downstairs etc. It's definitely bugged but the good news is that is doesn't matter. TBH I can't remember who I finally ended up letting talk to Roost, I (think) it was Ifan with Sebille downstairs, and even Ifan didn't get a quest update (maybe because of the elf captive quest, a theory) however both characters had their stories move forward at the Nameless Isle so don't sweat it.
