@thecrazyswede,
Your answer is close I think, but let me fill you in on a bit more information:
I have a technical background with so always trying to logically reason what goes on with my gear.
I had this issue for the first time today on my Lumia1020
I had this issue for the first 3 months of my Lumia920 before it was updated.
This is what I *think* happen(s)
The Lumia Cameras store the photo information into some kind of buffer (Ram?) So photo taken and written to Storage, Then "normally" the image is written out, with thumbnail and added to an internal database (Cache).
"IF" the database (cache) is not written out, no thumbnail is created or database (cache/index) and the camera eventually "fills" up its buffer (Ram?) and it hangs (locks) .
Hence you need to force reboot it, thus all the "buffer" is lost, so when it restarts the phone doesn't have any index for the pictures taken in the camera roll etc.
My reasoning behind this is that I have my old Lumia920 here and I have "MOVED" all the files off it to my PC and the photo's are still accessible from the phone, and I can't get it to purge the index (database), i'm going to have to do a full reset. (The strange thing is I can still open and edit the pictures as well.... gues that will keep working untill the file system is overwritten.... its very strange, its like file tables, one for Windows and one for phone internally)
On my Lumia1020 I only first experienced this today so i'm hoping its not a frequent event!
I post most of my pictures to instagram if anyone is interested.
Instagram Love my Lumia Phones!
I hope the team can find and fix the issues above! when I update my Lumia920 to black i'll try
My lumia1020 is already on black.