I had that with MHW and I nailed it down to shader (fossilize-replay - https://github.com/ValveSoftware/Fossilize).
From my guess, Steam support Vulkan shader pre-compilation so that you don't have to wait in game (like the infamous 10 min Monster Hunter Wilds startup delay). They also seems to also be able to download the compilation cache from Steam if someone already have done the process on the same GPU + driver version. Since fewer Windows games use Vulkan this feature is often not used, but on Linux most games will run on Vulkan (esp. Proton games with dxvk) you may experience the process more often.
Background shader pre-compilation does not use all cores by default and the only way to change that is to manually edit a file. So unless you're consciously changing it, you won't have this problem. It'll only use all cores when you launch the game.
I have been having the issue with the system hanging up when steam is doing big writes. I had assumed it was due to something wrong with my drive and was contemplating reformatting it.
This is a huge nitpick but I wish they'd just say "other Linux distros" instead of the "Linux platform". It's fine to pick and choose one (or a few) popular distro(s) to support, like SteamOS. It's not reasonable to expect support for all possible Linux software environments. It's already crazy that they support so many hardware combinations, even on just Windows.