I've also got this bug (Xbox One). I did create the fatigue potion before talking to Nurse Dorothy and also gave it to Mortimer Goswick before I was instructed to as well as a couple others who where suffering from fatigue. I also knocked on the door for Darius Petrescu a couple of times before going around to the blood pool on the side of the building to listen to the conversation. I could hear her talking but she was invisible. Just like others here 'Investigate Darius Petrescu's actions' didn't complete and I'm stuck on this objective.
Does the new update contains the fix for the bug?
Altough I restarted the game because I didn't want to wait for a decade for the fix to come and I got a very similar bug in the Newton and Oswald side activity, where I saw that i should investigate their secret dialogue however I cannot listen to it. I am at the right place at the canal near the iron fence but i cannot press X to investigate the dialogue... @Iyagovos do you have any solutions for these "investigate people's actions" quests?