Known issue of #Fairphone 6:
Camera pictures missing after capture.

This is really bad... I run into this all the time. Pictures I thought I had taken didn't actually get saved.

What's the best android camera app?

@JohnAZoidberg
There are two camera apps on the fairphone. The second one is built upon open camera. Did you tried this one? It is a bit more difficult to use.
I installed "libre camera" and you could try the camera app from fossify.
You can find both on f-droid.
Fossify is without exposure correction.
I haven't experienced this problem, but I will have an eye on this.
PS: I use the murena e/os version. With Play Store you might try "secure camera". The three respect your privacy.