This one stumped for a bit. You need to complete the investigation and one of them isn’t quite 100% working.
There is an interactable prompt in the lower left corner of the lower library room (the one with the Sophie pixel scene). This seems to be in a chair in that corner, ish. But it doesn’t show up with the sparkles sprite like the other interactables for that quest, you kind of have to rummage around to see it after interacting with Dog.
Once you complete that, the main quest is allowed to continue with the informant in the main hall