AFAIK, HDR was theorised to have been disabled due to problems on AMD GPUs, as said by a third-party driver developer (iVRy). They also said that issues with USB communication was why they weren’t able to do it properly either with the adapter having a more direct connection. The adapter should be able to support it when SteamVR is updated, so hopefully Sony releases firmware updates for it.

https://twitter.com/iVRy_VR (I’d provide a Nitter link but it’s playing up right now. Use whichever redirector extension if you’re that bothered.)