All this worry about RTH altitude control malfunction is easily avoided by just flying the drone yourself. Better to be the PIC than a spectator.
If you lose position awareness for the drone, it's dirt simple to follow the rubber band line on...
I would have no hesitation whatsoever in doing what you describe. I'd also be comfortable flying six feet above the lake en route to the location from which you want to shoot the photo.
Just don't rely solely on the altitude indicated on the...
Strange that in all the years I've been hearing this sort of nonsense, I can't find any recorded flight data to support it.
But I've found plenty that indicates the "plausible explanations" that you've listed above.