[mst]

It seems if the sun (or strong artifical light) hits the lens from just outside the image field at a particular angle, something in the lens creates this slight shadow on the image side. Works only on the short sides (but on both of them). My assumption is that it's the effect of a reflection from the rectangular mask covering the rear element of the lens.

Simulated backlight with the iPhone LED light:



Same with the sun as light source:





Also works with 'actual' backlight subjects:





The shadow shows up only at a particular angle, as mentioned. So, in practice it's probably not really a big issue, since it's difficult to hit that angle exactly and not too difficult to move a little bit to get rid of the shadow. Still, a reproducible issue of the lens.