Act 4 - Main quest advanced after killing Siren boss without quest item or dialogue

Just killed the Siren boss in Act 4 for the first time. When I did so I only had the first part of the weapon found from the abondoned prison. My quest required me to search for more parts.

The siren did not drop a quest item, however, after killing her my quest updated and said "speak to the hooded one". After I did so, my quest updated again and it now says "find the 3rd part of the weapon".

Problem is, I never found the second part of the weapon (at least there was no visual on screen like there was in the prison) nor was there any dialogue when I talked to the hooded one. it just updated the quest to "find the 3rd part".

Not sure if this has to do with the order I am doing the islands in but seems like a bug?