Both electric and propane are using the same set of t-stats for heating cycles so if t-stat was issue the electric would be going on/off along with the propane and water would not be getting hot due to the short cycling
The propane is cycling because it is NOT 'proving' main flame is lit (or proving it is staying lit)
That is a function of the spark electrode and grounding
Flame proving.....milivot signal from 'ionization' so circuit board keeps DC Voltage on gas valve UNTIL t-stat opens
Spark electrode assembly suspect.
Clean & reposition
