So, I’ve been having the same problem as many here with photos on the Fairphone 6 going missing seemingly at randomly, and found this page whilst looking for help.
I may have something else to test: disabling ‘AI scene detection’ in the camera Settings (cog) menu (you have to open the initial cog menu at the top right, and then the Settings option at the bottom of the popup - it’s the top option for me).
I tried this as I noticed I was specifically having a constant problem taking photos of the family dog. At a distance tended to be fine, but many closeup photos I took disappeared. The other day, I noticed a pattern - sometimes a small dog icon appeared in the top left corner of the phone when I tried to take a photo, and those photos always vanished (got stuck in ‘processing’ then weren’t there after).
After I turned off AI scene detection, I tried taking exactly the same photos again - no dog icon appeared on close up shots this time, and I haven’t lost a photo since. So it seems to have solved my main problem, time will tell if it works across the board. Might be worth a try for people?