I just had a look... with the monitor off, board power is around 8-9 W, which is awesome. Now, with the monitor back on, VRAM clock jumps straight to 909 MHz, and power consumption to 45 W. It's funny that it consumes less power as I'm typing this comment in Chrome than it does doing nothing on the Windows desktop, as Chrome lets the VRAM clock fluctuate a bit, while it's always 909 MHz on the desktop.
It was fine for a long time. I really don't understand what happened with these 24.x.x driver versions. I also don't get why reverting to an older version doesn't work.
Update:
I had enough of the issue, and put my 6500 XT into the system. It was fine for a while, but then I noticed that the VRAM clock was stuck at 1057 MHz during idle. I don't know by how much that bumped the idle power up, as RDNA 2 only reports GPU chip power draw, not the total board power. So then, I thought, whatever, I'll just live with this, and put the 7800 XT back. And now, it's fine(-ish). VRAM clock goes back into the 90-100 MHz range at idle, and power is at 24 W. What... the... hell?