I'm on Fedora 31.

I'm on max settings, and the game otherwise is perfectly fine.

Ok, for some reason setting it to "fast" fixes darkness xD

Can confirm. I'm having the same issue on Arch Linux with an RX460. Only happens on max settings.

Vulkan seems to work better and fixes the darkness with Good graphics quality.
Currently you can choose the renderer only with a command line argument like below.

./Karlson_linux.x86_64 -force-vulkan