Same Issue on PC as you can see i have 6/6 letters but the unlock req says i have 4/6
Exactly for me. However, if you put save data onto a USB then delete the save data on the console, not even the cloud or ps plus save storage. Just on the console, it resets the missions and you can go back an get the letters. THIS DOESN'T DELETE WHAT IS SAVED ONLINE. For example if you have completed the challenges you dont have to redo them. Just speed run grab them letters an be done. it resets the game progress, not the online progress. This worked for me.
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