The AMD Guy
Strange. My RX560 memory downclocks to 300Mhz but the core will only go down to 1.33Ghz. This doesn't affect thermals and the fans don't spin as you've described though. I'd assume you're using Whatevergreen? Have you tried using AGPMInjector.kext? You'll need to use the SSDT-PLUG.aml file so it'll attach to the X86PlatformPlugin though.