Reason:
Thermostat sensor control the heating start and off, if the setting is more than 55℃, it will start again when water temperature reduced 10℃, if the setting is lower than 55℃, it will start again when water temperature reduced 5℃, this can keep water in hot all the time.
Solution:
Water heater is normal, don’t worry.
You may switch off the unit if you don’t need hot water currently, and switch on just before you need it.