To the question of whether the factory reset fixes the problem in the long term: no.
I had the same problem with my mug for a year. Each time I performed the factory reset the problem would last me between 2 and 6 weeks. Black coffee would last it the longest. As soon as I allowed my kid to have a milk drink in the mug the problem would return within a few days. The sensor is not robust against any type of buildup. Its a hardware problem that cannot be solved by software updates.
I cracked open one of my Ember mugs after it died. The small pcb that is at the base of the mug has a sensing trace going up into the wall of the mug. Its capacitive, you can see the pad on the component. Nothing fancy.
