There is not a lot of information about how data is saved in the background while working with FileMaker Go 15 or the SDK 15. What I have been told by support is that data is saved all the time incrementally while working unlike the desktop version.
If the FileMaker file is not closed while the FM app is in the foreground it might be purged by iOS at any time while the FM Go app/SDK is in the background and iOS needs more memory for the current foreground app.
To my knowledge the FM Go app/SDK does not distinguish between a crash and a purged file. Hence when re-opening your file the FM Go app/SDK might revert to one of the automatically saved versions that was available prior to the purge.
Yes... I believe, that it could be an Apple issue, not (not only) FileMaker. That's about the same that one member of the local FM group said last week. Anyway, losing data is not what one loves
I remember that at the time when I worked on the other app, the connection failed (LTE network), but the FMGo file is on the device, no server needed, no read from url, no webviewer,