Originally Posted by Spoonrobot

I suspect this is the cause. I service some bike share bikes that sit on new black-top, they had the same problem last July. The rims were showing temperature of ~135°f with a contactless thermometer around mid-afternoon and the tires would flat later in the evening. Same exact visual appearance as riverdrifter posted. Used some gorilla tape to make a buffer around the valve stem hold and the problem was resolved.