Puzzling double image/reflection with Canon EOS R

[nardes]

Here is a photo of an Australian Grebe in deep shade and backlit with a low sun, taken with the Canon EOS R, Canon 100-400mm F4.5-5.6 L IS Mk II with a shoe mounted Canon Speedlite 600EX RT in HSS Mode.

SS=1/800 sec, F9, ISO800 at 400mm focal length. The bird was darting around and diving quickly, hence the 1/800 sec as 1/640 and lower produced a blurred result.

Notice the “real” smaller dark shadow which I assume is the natural shadow from the low sun behind the bird, then there is the fainter, semi-transparent outline shadow, almost a mirror reflection of the outline of the bird overlaid by OOF highlights in the water.

Weird to say the least – is this some kind of High Speed Sync artefact from the on-camera Speedlite?

Note the AF missed the bird’s head.

Cheers

Dennis