I read the comments on this forum (in this post and others) as well as the comments on the DJI Forum regarding the SPARK and distance issues.
DJI has a legal disclaimer in their add-on manual that says pretty much anything electrical may cause interference so pilot beware. That is their way of saying "I know we said it could fly over a mile, but only in optimal conditions".
It is true that the SPARK will fly (out of the box) up to a mile, and perhaps farther, if all conditions are good (i.e., interference is at a minimum).... BUT.... what is not written in the literature is that distance is based on flying pretty much in a straight line. If you decide to change course once you're past half a mile, you may lose video or telemetry.
I had not flown my SPARK much past 300 meters, so decided to try and see if it could fly up to 1600 meters (1 mile) in distance. I selected a spot over farmers fields where there would be few electrical signals (power lines, WiFi, Cell Towers, etc) to cause interference. For interest, I did leave the Blue Tooth enabled on my phone and left it connected to my FitBit. DJI says even a FitBit can cause interference because it is sending a BlueTooth signal.
In the end, I did pretty much make it to a 1 mile distance and the video signal was not bad, although it was becoming less fluid near the one mile point, and then suddenly the signal became lost & RTH self initiated. To the SPARKs credit, it flew back and landed within a couple of inches of where it took off (most impressive). Also since I flew out in Sports Mode, it appeared to return to me in Sports Mode (something I didn't expect).
One last thing... I did not use an OTG cable. As I understand it, the OTG cable is more for when you are in an area causing interference between the controller and phone, and not the controller and drone. Perhaps I'll try it in future as a MYTH or FACT distance video.
Here is the video of the event.