well i think you have a faulty driver install - or your HardwareVideoDecoder in your has broken
1. update Win10 to 20H2
2. disable Win10 auto-driver-install
3. run DDU in safe-mode to clean old driver
4. install latest AMD driver from website (dont use auto-detect-tool)
5. test again
Try eeset factory defaults and reset the shader cache under graphics, advanced.
Have you tried a complete driver uninstall with the AMD removal tool and then installing again?
Also check all power connectors and reseat the Video card.