Idk what llvm is. Here is clip of the issue, uploaded to Firefox Send:
https://send.firefox.com/download/dcd78ddb8c/#gRGpMF2-xa5eX62FpOBTJg

Interesting. Please post screenshots of that in the bug as well.

To check what llvm Mesa is using, please post these:

glxinfo | grep string

vulkaninfo | grep deviceName

glxinfo | grep string
server glx vendor string: SGI
server glx version string: 1.4
client glx vendor string: Mesa Project and SGI
client glx version string: 1.4
OpenGL vendor string: X.Org
OpenGL renderer string: Radeon RX 580 Series (POLARIS10, DRM 3.23.0, 4.15.0-43-generic, LLVM 8.0.0)
OpenGL core profile version string: 4.5 (Core Profile) Mesa 19.0.0-devel - padoka PPA
OpenGL core profile shading language version string: 4.50
OpenGL version string: 4.5 (Compatibility Profile) Mesa 19.0.0-devel - padoka PPA
OpenGL shading language version string: 4.50
OpenGL ES profile version string: OpenGL ES 3.2 Mesa 19.0.0-devel - padoka PPA
OpenGL ES profile shading language version string: OpenGL ES GLSL ES 3.20

vulkaninfo | grep deviceName
INTEL-MESA: warning: Haswell Vulkan support is incomplete
        deviceName     = Intel(R) Haswell Desktop
        deviceName     = AMD RADV POLARIS10 (LLVM 8.0.0)

Any guideline for how to properly report this issue?
Also I see in second output it says "Haswell Vulkan support is incomplete". Could that be the cause of it?

EDIT: The game is playable but it's such a bummer that this happens :(

Your kernel looks pretty old: 4.15.0-43. Can you try installing latest kernel and run the game using that?

Intel Vulkan shouldn't affect the game, unless for some reason it's running through anv and Intel GPU.

You can see what is being used in the dxvk log. Or run the game with dxvk hud enabled, by setting this environment variable for example:

export DXVK_HUD=devinfo,fps,memory

It's one that came with Kubuntu 18.04 so I didn't install a newer one. Any tips on how to do that safely? I briefly viewed a video by (Level1tech?) where he talked about a Ubuntu program that helped with it but thats all.

EDIT: It's not running through dgpu or anv. It runs at 60+ fps at Ultra.
I made a error thinking the incomplete line was maybe relating to non graphics aspects of the cpu.

Quoting: LinuxwarperIt's one that came with Kubuntu 18.04 so I didn't install a newer one. Any tips on how to do that safely? I briefly viewed a video by (Level1tech?) where he talked about a Ubuntu program that helped with it but thats all.

I haven't used Ubuntu myself, but as you say, there is some standard tool that helps managing kernel installations from the official Ubuntu repo.

You can also simply do the search for available kernels like this:

apt-cache search linux-image

Find latest 4.19.x or even 4.20.0-rc7 and use that.

Make sure you are also using latest amdgpu firmware. I install it by hand for example, since in Debian firmware-amd-graphics package is behind upstream.

I haven't made changes to kernel because I don't like to cause instability. What is the probability of it being the issue?

The bug issue page is locked btw.

Probability is very high. That kernel is ancient when Mesa is concerned. Mesa relies on amdgpu kernel driver, and that is as recent as your kernel is. I'd say, first install newest kernel and newest amdgpu firmware, and see if the issue still persist. If it does, then open a new dxvk bug.

Kernel 4.19.11
The issue persists. Should I stick to that kernel or revert? What I meant earlier is that I don't feel confident that I can fix things if most up to date kernel breaks them.

You can stick to it. It's highly recommended for Mesa usage in general to use latest kernel anyway.

Feel free to open new dxvk bug, so developers could investigate.