I have more information on this bug, which is not yet resolved. Several things don't add up at once if you take the scenic route to this quest.
My progression:
-Explored the old city first.
-Found the cornet of depths, and installed it in the wall after the boss fight
-Some time after, my reputation in the gullet is high enough to get called to Dereo, which starts the quest.
-I don't remember the exact wording, but at this point it seemed clear to me the dialogue line only acknowledged my acquisition of the cornet of depths, not my possesion (it was already socketed into the wall), so here's probably the first miss that make everything go awry.
-Nonetheless, dereo sends me to acquire the cornet of waves. He also gives me his flag.
-In the dialogue with it's owner, i can show off my cornet of depths even though i no longer possess it. If not before, this was a clear sign of trouble.
-I stole the cornet of waves.
-Dereo does not recognize this feat - there are no dialogue options related to the quest. I should've looked at the journal at this point, but forgot.
-Installed the second cornet in the wall.
-Met this random person in the temple talking about dereo.
Also, closely related, there's trouble with the lift in the row. If you take it up from the smugglers without Dereos´ prior approval, there's no proper handling for this. They just shoo you in the direction *you actually want to go* because they only know how to prohibit an adventurer coming from the south. If they tell you not to pass, you still can. Coming from the north, there should be responses reflecting on this based on what assumptions the guards could make from you coming unexpectedly from that direction.
