I have media-libs/amdgpu-pro-vulkan installed and set VK_ICD_FILENAMES to use the radv mesa driver. Regardless of the env variable gamescope attempts to use the proprietary driver. Compiling gamescope manually behaves as expected.
I find that surprising. Please provide logs of your Portage build and your manual build.
Created attachment 851162 [details] portage build
Created attachment 851164 [details] manual build log
Works fine on the latest version. I still have no clue what caused this, but the only change was updating the package and now it works.