System Information
Compatibility Report
I can confirm problem with the shadows (occurs both with LLVM and ACO).
However the problem is not visible in 1920x1200 or 1920x1080 resolution. Or if you start the game in a higher resolution, switch to a lower and then back to higher.
Here I started the game in 2560x1600 resolution and the game looked like this:

Then I switched the game to 1920x1200 resolution and the stretched shadows went away, and then switched back to 2560x1600 resolution:

As you can see the stretched shadows on the rocks are now gone, however I suspect the picture below is not actually correct rendering either (I think there is probably some shadows missing that should be there).
System Information
Retest Compatibility Report
The same issue with stretched shadows remain with Proton 5.0-4. I tried also launch option wined3d and got a different rending issue (white textures).
Out of the box:

With WineD3D:

The shadows looking awful is actually also a problem on Windows when you use resolutions that the game doesn't support natively (i.e. ones that aren't in the Resolution slider in the Options).
The workaround I found from this Steam post does work however.
To apply the workaround you need to create a user.cfg in your game's location and populate it as that post states.
As an example here's what's in my user.cfg forcing a non-native resolution (as well as fullscreen due to weird menu / scaling issues) and applying the shadows fix:
r_Fullscreen=1
r_Width=2560
r_Height=1440
e_ShadowsMaxTexRes=512
r_ShadowJittering=3
Screenshot:

Addendum: Using Proton-5.5-GE-1 on a AMD 5700 XT (mesa-git-68f325b256)
proton 5.5-ge-1x1 2020-04proton 5.0-4x1 2020-03proton 4.11-3x1 2019-08
Compatibility Report
System Information
I confirm:
steam-329430.log
Symptoms
It seems that everything works, only the shadows are stretched.
Demonstration: https://youtu.be/926cLHrd9Jg
Renderdoc here: https://mega.nz/#F!p14w3a6K!epF-_v1FawPhmeZcc-9cEA
Can anyone look at what the problem is?
Reproduction