Is there a second room thermostat somewhere in the house by any chance and yes, if no thermostat control for room temp via Programmer, then this is most odd and what Microsoft might call ‘A feature’
Nest Thermostat-E probable being temperamental 
@Grisu, Not everyone can afford 21°C, or they might be very carbon conscious 
We have a baxi boiler and a couple of Room stats in different locations with an dedicated Programmer for timer for hot water and heating etc.
We also have a hot water tank, which has its own thermostat, set to around 65°C (ish) I think
In a completely sane world, madness is the only freedom (J.G.Ballard).
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