We have also encountered this error. It seems to be prevalent on iOS devices, and it rarely occurs on Android devices. We have tested iPads, iPhones, and a variety of Android phones (mostly Pixel phones). We actually took our iPads out of circulation for S123 use in the field. It was happening that often. And it was just as you have reported. A user takes a photo, then views that photo to make sure that the composition and content are good, and then taps the back button to return to the form. This often results in nothing visibly happening on iOS devices. However, if the user taps on the back button again *poof* the survey is gone. Crashed to desktop (or in this case phonetop) without so much as a whimper. Any data recorded for that form that was not previously saved was irrevocably lost. On a few occasions, users did report a recovered survey screen. But most times nothing. They reopened S123 to find their entire form had vanished. Most of our forms have the photo at the end of it, too, meaning they had to fill out the entire, multi-page form again.
In my own tests, I discovered that if one is using an iOS device, and really needs to view a photo, one could do so under the following conditions: Great care must be taken to tap the back button only once, and if the photo doesn't close and return to the form, one could tap nearly anywhere else on the screen aside from the back button again to return to the form (I generally go for the center of the photo itself). I have used this workaround many times, and have only absent-mindedly tapped the back button more than once. This is not ideal, though, and should certainly be addressed.
Thanks for posting this. I have been meaning to make a post about it myself, but had not yet found the time.
