Well, that's the major issue. Normally, the quest works just fine. I've played through HoS on two different playthroughs without issue. If the bug pops up, it's still not understood what in the gigantic pile of exponential variables is causing it. Unless the steps can be identified to make a bug occur reliably every, single time, then correcting the source of the issue becomes guesswork.
The idea of making the diagrams available some other way has been suggested numerous times before, but unfortunately, it's not possible to edit special items into the existing inventory of merchants without utterly destroying the whole system. (It's not as simple as just "add this item to this merchant".)
Myself, I believe it may be somehow related to the inventory overload issue. If item IDs become too large as the game is saved and reloaded, or if the game happens to generate an item ID that already exists in Geralt's active inventory, weirdness may ensue. It seems the more that needs to be loaded into RAM at once, the more likely it is to hit an issue. One of these "weird things" is items turning "invisible". Meaning, data references still exist, but the item itself will not be represented by an icon. It seems possible that this might be what's happening with the "missing" letters and schematics. If they were truly gone, the game would likely crash. Since the chests appear empty, it may simply be that the game hit a snag (based on the literal millions+ of possible inventory combinations a player may have at that moment). Ensuring that no background stuff (friends features, internet, etc.) is running in the background may free up enough system resources to just bull through it. (Just ensure that you reload to a point before you actually started the quest itself. Cleaning out your inventory to the stash and taking only what you need would be a good idea, too.) Might be worth a shot, although I haven't heard of this working for anyone yet. Wish I could offer something more!
