1. I placed the scroll in the chest in Vendicci's manor without being seen.
2. I got the journal entry about the chest flashing brightly and I used divine intervention to exit the manor.
3. I returned back to Aetia Nemesia but Merxia Vendicci was not standing next to her as she was supposed to. This is the bug.
4. Going back to the Vendicci manor made her appear next to Aetia and I could continue the quest.

I had unlocked all the doors and stolen everything from the chest before starting the quest.