One of the things cited in some water heater manuals is to check for under-voltage if water is cooler than anticipated.
Like David said, test for 240V across both screws on breaker (or across top two screws on upper thermostat), and then move water heater wires to another circuit breaker to eliminate breaker as suspect.
If you have a cracked element shorted to ground that doesn’t trip breaker, then the element would not turn off, and element would continue heating with 120Volts flowing to ground wire at top of tank.
At some point, both elements could be ON at same time.
This could cause breaker to heat up and cause arcing at busbar > resulting in possible circuit breaker problem, under-voltage and cooler water.
Element shorted to steel tank
This is why each element screw has to be tested to steel tank. There should be no electrical connection between element screws and metal tank, or element is bad.