When SteamVR is closed the attached HMD no longer powers off. This is due to the display still being driven by the GPU.
The first bad commit for this behaviour is:
"drm/amd/display: Allow commits with no planes active"
My understanding is that the commit above does fix a bug in amdgpu/dc. However, due to current Xorg versions failing to shut down outputs after re-claiming a drm lease, this commit ends up breaking userspace.
Following is the x server patch to help illustrate what X is doing incorrectly: