i’m not an expert on hot water heaters, but is it possible its the difference of upper vs lower element?
My understanding of electric hot water is that the upper element usually fires first to heat the water closest to the outlet. Then when that hits temp, the lower element turns on to heat the bottom of the tank.
So in a hot water heater that is being actively used, the top element does the heavy lifting until the water demand stops and it can catch up the upper part of the tank, and eventually the lower element turns on to finish off the heating cycle.
For a tank that isn’t used, presumably the bottom of the tank is going to cool before the upper portion of the tank does, so in that scenario, the lower element may turn on out of sequence to what Sense has id’d as “Hot Water Heater”, so the Sense doesn’t ID it.
This could be a terrible suggestion, but just a thought.