This is also can be reproduced with dxvk legacy,
so I checked some versions of NVIDIA drivers and I got the following result:
530.41.03 - broken shadows525.116.03 - broken shadows515.105.01, 515.86.01, 515.57, 515.48.07 - broken shadows510.108.03 - shadows looks fine470.182.03 - shadows looks fineUnfortunately, the problem occurs somewhere between the 510.x and 515.x branches (I can't seem to be more precise).
upd. I did more tests for a more accurate result - information has been updated
The initially described issue is not present on DXVK with NVIDIA 470.182.03 and 510.108.03 drivers.
proton8 on 510.108.03:

dxvk legacy on 470.182.03:

Noting that i reproduce on my RTX 4080 with 550.40.61 and dxvk master
proton 8.0-2cx1 2023-05
Shadows break when shader quality is set to "High", "Medium" or "Low". On "Extreme" or higher - all fine.
(so presets "1080p High", "1080p Medium", "720p Low" are enough to reproduce the problem)
screenshots of the problem
How it looks on dxvk:


How it looks on d3d11 native:
Highly probably NVIDIA specific issue.
Software information
Unigine Superposition v1.1 (https://benchmark.unigine.com/superposition)
Shaders quality "High" or lower (on "Extreme" or higher - all fine)
System information
Apitrace file
*internal monitoring was flickering already during apitrace capture
Proton log file