After years of seeing this I finally had it happen last weekend.
The "cause" or, the only thing I did differently and knew never to do, was to take the phone out of airplane mode while onX was open, and service was spotty at best. The "fix" for me was to get to a place with good service, open onX, turn on the lost layers and let them populate, then close onX. Put the phone back into airplane mode, start onX and everything was back to normal.
Right or wrong, I have a rule that onX never gets started unless the phone is in airplane mode (when away from home), and onX closes before the phone come out of airplane mode, always. If you have good, constant service it may not matter, but if it's spotty to non existent, it causes problems. This appears to be unique to onX, I haven't seen it with the other vendors.