This is because there is a +/- 0.5º deadband on the system so that the rooms maintain a stable temperature.
To solve this problem you can set a positive valve hidden offset on the valves in the rooms affected, to raise the temperature the valve controls the room to. This will turn the boiler off once the room overheats by more than 0.5º:
Genius Radiator Valve Offset Temperature.
Why do we do this?
When a room is heating the boiler needs to stay on so that the valve can maintain a stable temperature in the room. The valve modulates exactly to allow just the right amount of heat into the room to keep the room at exactly the temperature you set. See the accuracy of the valve here: Genius Radiator Valve controlling the temperature accurately and not overheating the room.
If the boiler is allowed to shut down during this time, though it may appear that this will save energy the problem is you get an undesirable temperature drop in the room and this causes inefficient temperature swings in the room. This is because by the time the valve has responded to the temperature falling too low, and the boiler re-started, all the pipework warmed up, and the radiator warmed up again, you will now be in a room that is too cool. To save the most amount of energy the room needs to be at a stable temperature and the one that you set on the app. This is achieved by the valves allowing just the correct amount of hot water into the radiators that the room requires.
However, if a room was to overheat (because of a secondary heat source like a log fire in a living room or an oven in the kitchen) then we will keep the boiler off until the room has cooled sufficiently. I.e when a room is cooling (for example the measured temperature coming down to the 19.5º), with a target temperature of 20º, this would only come on (and the app turn orange for that zone again) once the temperature drops below 19.5º.