I have RX 590 (Sapphire Nitro+) and one single monitor MSI Optix MAG24C (1080p). Monitor Displayport connection has some minor problems (related to monitor) so I have GPU connected to monitor via HDMI. And now the problem: when I set refresh rate to 120Hz or 100Hz, GPU memory clock does not downclock to 400 MHz and stays at 2100 MHz when idle. Only if I set refresh rate to 60Hz, memory downclocks to 400 MHz. But I have 120Hz (HDMI) / 144Hz (Displayport) Freesync monitor and do not want to run at 60Hz. More interesting is, when I tried to connect monitor via Displayport, at 144Hz memory runs at full speed, but at 120Hz memory properly downclocks to 400 MHz. So the bigger problem is via HDMI port. The card runs uselessly hot and draws more power when memory runs at full speed all the time.
So to sum up: Displayport at 144Hz, HDMI at 120Hz and 100Hz - GPU memory does not downclock when idle
Latest drivers 19.9.2
edit: I have played with custom resolution and refresh rate (in Radeon settings), and highest refresh rate where memory properly downclocks is 88Hz. At everything above 88Hz memory stays at 2100MHz.