If there is a correlation with the device going into power save mode, please reach out to Garmin Product Support so we can research this.
There is a correlation. I have had this happen to me twice: Long ride, with radar. Device offers to go into power save near the end of the ride. I end my ride. Everything appears good. Right before ending, my totals all seem reasonable. I hit Save Ride.
But my ride will never upload. I can see the ride in my history. But I can't open the ride.
If I mount my device and copy the .fit file out and try to manually upload it to Garmin Connect, it tells me that it is an unsupported file type.
This is really frustrating, to not be able to see your data fro your most impressive ride.
If I had to hazard a guess, I would say that hitting Save when the device is in Power Save mode is asking for trouble. My suspicion is that it is powering down in the middle of saving the data, leaving the .fit file incomplete or corrupted.
Happy to send the most recent example of a failed .fit file if that will help you diagnose the issue.
Note my device is an Edge 820