Another avenue of investigation that comes to mind based on your describing that the GPS lock is good to start with, but then deteriorates, is to check to see if the iFly GPS app is being managed by the battery optimization part of the operating system.
On my Samsung S21+: go to Settings | Battery & Device Care | Battery and make sure Power Saving is turned off. In the same screen, tap on Background Usage Limits, then see if iFly GPS is in the Sleeping Apps list or the Deep Sleeping Apps list. If so, long-press on the app icon, then choose Remove.
Also: go to Settings | Apps, find iFly GPS in the list and tap on it, tap on Battery, and make sure the Unrestricted radio button is selected.
I had an issue with MyFlightBook being flaky (GPS-wise) on my new phone and battery management was the cause.