Key fragment missing - Tomb of Fariel

I have searched all five tombs. I solved all the puzzles, all tombs are fully open to me and there is nothing left to loot in them. I've been back and checked each tomb twice after the initial search. The quest says I only have 4 out of 5 fragments.

I've been to the door of Fariel's tomb, past the quietly snoozing dragon, but I can't get in without that final key fragment.

At this stage, I can only assume it is a bug, but I would gladly accept any advice. The internet only seems to have answers for people who haven't found all the tombs. (Yes, I checked the first tomb right by the camp. I actually distinctly remember getting the fragment from that one!)

Are any of the fragments hidden away somewhere unexpected?

Does anyone know if this as a bug that can be fixed?

I'm playing on a PC (Windows 10), at this point if there's something I can edit  to make the thing recognise I have all five fragments I'll go for it. There are hardly any quests left in the game and I want to go for the endgame, but I'm a completionist. PLEASE HELP.