We have had this issue reported as well, in the past, but not pinned to a particular device. We also us a combination of Android and iOS devices. It is hard to document this and the normal suspicion of the reviewers is that the field user must have done something wrong. We don't store the new data on a hosted service, but to our own ArcGIS Server, but we have still had a couple reports of this kind of "lost" data.
On recovering the photos from the hosted data, good luck. Even your system admin won't have access to the ESRI servers on the level needed to explore that. Not sure if you could get the ESRI team to explore that by opening a support call, but it might be worth a try.
We instruct field staff on attaching photos to take the photos from the mobile device camera app prior to starting the new point data capture in Field Maps, so that it gets saved to the device camera roll. Then select that photo from inside the Field Maps data collection process as opposed to snapping the photo from inside the process where it does not save to the devices camera roll. When everything works right, that is redundant and takes a few more clicks, but you also always have a backup photo. Plus it is far easier to share photos from the camera roll for other uses compared to the way you retrieve the photos from the hosted database.
Hope that helps a little. I am interested in seeing what others say.
