You’re right the top element turns on first when starting with a cold tank. Once the top of the tank reaches temperature, the top element turns off and switches to the bottom element. From there on, as long as the top of the water column stays hot, it is the bottom element that switches on and off.
The top element usually never cools down enough to turn on again unless there is a large draw down of water and a lot of cold water floods the tank.
The bottom element is the workhorse of the tank.
A number of people, me included, have seen where Sense seems to report half the power draw of heater, almost as if its reporting only one leg of the 220v, It could be that the upper element turns on and it is lower power than the bottom element. But Sense reports almost exactly half the power so it seems it really is only one AC leg being reported and not the upper element.