After reading the ways you're supposed to find these answers, it does make sense now. I can absolutely see how not knowing to close the door when you have the black light be an issue.
I would say it's more important to have a hint telling the player to look at the letter once they DO have the black light in place than the one about the PIN code to the articles. I figured that part out really quickly, and it was more than I didn't know the black light revealed the numbers. I had otherwise figured out the PIN was related to the article and was only missing what specific parts. In the end, I ended up brute forcing the last number (once I noticed every single article in question HAD a number on it somewhere, all of which were just single digit numbers save for that last article, I had it in the bag. Then I just went in order). Having gone back and seen the how it looks with the black light, though, it easily makes sense. However, I feel like I wouldn't have figured out the code in the letter as I never would have looked at it again.
Also, your thanks should go more to Chef of Indie Questing and to Altabe Studio than to me. They're the two that handle all of the organizing. My part in this whole process is that I make the attempt to ensure that everyone's game gets coverage and everyone is able to get feedback from at least one person (aka myself. Also helps me flex my game design thinking, even if I lack the skills needed to make my own games. Maybe one day, lol).