During the Journey to the Past investigation, I spent ages running around Pembroke trying to find the third letter, before giving in and looking up a guide on YouTube. I went to the place the box is found in the video (Storage room, second door on the left through the main entrance, behind a partition), only to find it's not there. I've tried resting and coming back, and restarting the game. Neither worked.
Is there any known fix for this? If not, will it be addressed in an upcoming patch? (Story wise, I've just defeated Doris Fletcher, if that makes a difference).