It should go down to around 13w without having to leave. Mine doesn't even do that haha, just stays stuck at 30w. I'm not too bothered to be honest, I know it's a bug though as if I set 60hz it drops to idle power almost straight away if doing nothing.I'm using a 9070 XT with a 4K 144Hz screen over DisplayPort 1.4, at idle my memory clock is at 909MHz and while browsing the web it frequently goes up to 1538MHz. My power consumption is a bit lower though, at 909MHz it uses 22w and at 1538MHz it uses 26w. If I lock my computer and leave it idle for several minutes the memory clock goes down to 75 and the power consumption is 13w.
My XTX had a similar issue for a while until AMD fixed it.
There was a workaround where you set a manual display resolution and output. I cannot remember where I found it last time though.