It seems to be related to oiled porous wood surfaces and the HomePod's silicone base. Still odd that this would be missed.

https://www.imore.com/how-to-stop-homepod-from-leaving-white...

Given how quickly Apple has admitted to the issue, and the existence of their “recommended cleaning” method, I wonder if they did.

I've put it on this for a couple of days and haven't noticed a white ring: https://www.cb2.com/framework-credenza/s214354