It’s possible that you’re having the same issue I’ve had….someone else just mentioned it as well….
If you open Legend on iOS/mobile and leave it open after making some changes, it will stop syncing somewhere between a few hours and and a few days.
If, while it has stopped, you make changes to your data, and assume it is saving them you will be incorrect. If you go and make changes on your PC (or other non-mobile platform), they will sync, and seemingly delete the data you entered on mobile (because it never synched.
This bug has yet to be addressed, so now I make it a habit to enter data on iOS/mobile, scroll up (forcing a sync?) then I force quit Legend (so it cannot stop synching on it’s own). Since I started doing that I have stopped having sync issues that involve iOS.
That’s been my workaround.
Also, check my past postings about iOS and iPhone syncing issues.