[Klaus]

I've posted a couple of test shots here:

https://drive.google.com/file/d/1CU1AH4w...AMmT0/view

Both same speed (1/4000s) and aperture (ƒ/1.8), ISO 100, a6700, mechanical shutter and electronic shutter.
After these tests, things are even stranger: the dark banding at the lower side is not present with the electronic shutter... but it's at the upper border. In previous tests I didn't notice it.

I finally found these images ..... have you tried taking these images at a lower shutter speed to see if this changes the dark areas? 1/4000s may be the source of the issue ... just a thought!

This kind of vignetting is due to the shutter and has nothing to do with the lens.
Shoot below at 1/500 or slower, and you won't notice this anymore.