Fedora does test everything before they ship it. Each major kernel release can go through as much as a month of testing for stability and regression. SteamOS is based on Arch, where they don’t test the kernel for regression. Despite testing though, this is an incredibly obscure issue, and obviously the Fedora team can’t catch every kernel bug. It only happens on some hardware, and only in the event that the VRAM visible to the CPU is filled, and less used portions of the CPU-visible VRAM are moved to other parts of VRAM that only the GPU can see. This is why resizable bar fixes the issue for many, as it makes all VRAM visible to the CPU, so there is no move that happens (moving the VRAM data has an off by one error). This issue goes all the way back to 6.6.30, and was only discovered 3 weeks ago, and took 2 weeks to find the root cause of and patch in the stable version of kernel 6.9. It was only found because the 6.9 release candidates added checks for hardware capabilities, and the off by one error that is the root cause of this issue threw an error with the hardware capability checks. I’m not a kernel developer, so I don’t know all the details, but it is discussed in the issue I linked if you want more explanation.