Why do these images have a dark semi-circle on the left side?

Question

I shot these images with my Sigma 24-70 and Sony A7iii, in fog, sometimes pointed into the "sun" (it was not actually sunny but sometimes the lens was pointed towards the direction of the sun). I believe they were shot at above 1/1000th of second. I have front e-Shutter disabled.
I've only ever seen this happen with Sigma lenses (24-70 and 85) on my A7iii, not other brands. I'm not saying it's not possible, just that I've never noticed it. It seems to happen irregardless of the lens hood being attached or not.
I'm wondering why there is a darker shadow on the left side of the frame with a semi-circle/halo-like pattern there.

Kai Mattern · Answer

Sorry for the bad news:
Semi circular artifacts especially on a fast shutter speed are usually an early indication of shutter failure on the a7 III.
Examples:

Please inspect the edges of your shutter curtains for damage. The shutter edges should not be rough in any way. Often on the a7 III the shutters starts serrating which is reported as more sensor dust. Then the chance increases that these serrations start to catch on some other part of the shutter which will then destroy the shutter for good.
Example of serrated shutter curtain:
Metal shaving from shutter box of A7iii?
Cost for replacing the shutter is approximately $300. If the camera is still under warranty, you better act quickly.
See also here for similar effects, which have been reported back as shutter damage. https://www.dpreview.com/forums/thread/4286654

Why do these images have a dark semi-circle on the left side?

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