Each element has it's own thermostat.
They sell them as uppers and lowers.
The power only goes to one at a time.
Therefore, if something is wrong with the top, you will get no hot water.
The top must satisfy before power switches to the bottom.
The bottom can go bad, and you can still have heat at the top,
If that happens, you will have reduced temperature or volume of hot water.
hj, I'm changing in the van if I need to, or I do it so fast you can't see me.