I can confirm that turning off device polling fixed it, thermal widget workes as expected now. Explains why loading a config showed the issue, device polling is off be default, config kept turning it back on.
Also a positive, my CPU would hover idle right around 13-15% load, all the time when device polling was on. It is now idle at 0% with device polling off.
Weird though, device polling didn't have an issue running in 2.1.x…