Same Issue with my 5700xt, only way I could get it to idle at 200 mhz is turn off my second monitor My setup is main 1440p 144hrz and 1200p 60hrz monitor. This also tied into overheating, screen flickering while gaming with 2nd monitor on and game crash /hard restart soon after even with a very aggressive fan curve. I also noticed with my second monitor on the junction temp would be 30c hotter than gpu temp, once I turned off the 2nd monitor temps even out. Even with 2x 1200p monitors still issue so its not the 1440p.
I found a compromise, but it has drawbacks. Same card by the way.
Create a custom resolution in Radeon Software at 142hz refresh, choose CVT Reduced Blanking and hit save. Don't forget to apply this through Windows Display Properties as well. I use an LG32GK650F with DP cable.
The drawback is that BPC (bits per colour) won't go above 6bpc, but VRAM idle goes down to 200Mhz and temps by about 10c.