Since that bug still occurs, I can propose, at least for PS4, a solution which worked for me:
-Get a usb stick. Go to settings=>saved data management
- put the usb stick in and copy your saved data from storage to usb
- put the usb stick out an delete saved data on storage and online Storage both
- start the game, you'll have to adjust brightness e.t.c... you can quit the prolog from Narvik on. Then go to war stories and start the mission on any difficulty. The game still shows your previous achievments, that's normal ignore it.
- now the letters will be there you can get them. You won't have to do the challenges you already completed again! You can quit when you got the letters, to be sure do it when you're at the cathedral.
-put the usb back in and copy your saved data to the ps4 storage. Overwrite it yes.
- now you'll have the 6/6 letters and if you've done all the other challenges, the tiger skin
PS: it's possible to get the last 3 letters without fighting. After you shot down the planes, take the bazookas lying around to take down the shermans an kill all soldiers, don't enter the Tiger! Now you can simply walk to the letters, the game won't load any enemys now 😉 afterwards just get back in the Tiger an complete the mission normally
