@ChuckPa Yeah it must be something to do with the new Kernel then 
@ad878 you have the same symptoms as me right? was it all working before the Unraid 6.12.x update to the new Kernel and then after the update it stopped working.
Im 100% sure everything is correct on my side because it was working perfectly before. The only thing that has changed is updating Unraid which has the new kernel. I have checked and re-checked all settings multiple times and the iGPU and everything else is all accessible and working fine.
I have a backup server which is a Gen 9 (9600K) chip on my other Unraid Machine which is also updated and using the new Kernel and that works fine. Its just the Gen 12 that has the issue and the only way to fix it is to either roll back Plex or disable Tone Mapping and then HW Transcoding works.
Others are having the same issue as i am but they are waiting on this topic thinking it might fix the Alder Lake issue 1.32.2.7002 which by the looks of it, wont fix this issue.
I think ill have to patiently wait for the Plex team to update their Kernel and/or Unraid box to the latest version and im sure you will encounter the same issues as us then on Alder Lake.
