Rainbows always appear directly opposite the sun. As the drone rose above the physical horizon, the rainbow did, too. With only a little more lift, the circle would have been complete.
The drone view illustrates another common yet frequently overlooked aspect of rainbows. They are double. The interior or "primary rainbow" is caused by one reflection inside raindrops. The exterior or "secondary rainbow" is caused by two reflections. A double drone bow is a rare sight indeed.