Do you turn off your heating when, for example, you go on holiday in the winter or is there a window open in the winter? Does the heating start anyway? Or does the zone not have to heat according to the schedule and the heating still goes on?
Then it is possible that this has to do with frost protection. This happens when the valve detects a temperature below five degrees Celsius. This will prevent your pipes from freezing.
When frost protection is switched on, the display shows "FROST"