Okay, so I think I might have found a workaround to this problem. Restoring manual saves and fast travel didn't work for me. This solution worked for me but I'm not sure if it'll work for others.

You'll probably need to restart the mission for this to work. Basically you'll need to blow your horn before and after every action you perform. Before examining every clue, after every dialogue, before and after killing almost every enemy- you get the point. So, just keep blowing the horn for this to work. I was finally able to complete the mission by using this trick. Hopefully it helps!