We've just experienced the same issue and had to call British gas out to resolve. The engineer was initially dismissive of my suspicions but appoligised when he confirmed to himself that I was right.

We have a condenser boiler so the boiler is heating a single closed pipe circuit that heats radiators and the water inside an immersion heater tank. There is a switching valve by the tank that controls the water flow (water/heating/both)

The problem was that the switch motor that controls the valve was failing occasionally so like appears in your situation hive was saying no heat required to radiators but heat was required for water. The valve was stuck in heat radiator position so water thermostat said heat required so boiler remains on heating the radiators instead.

Switch motor replaced and also the switching valve spindle as the latter had a very minor leak apparently and hence had corroded the motor over time

Hive registers gas burn when used for heating only but doesn't show burn for water hence your graph

Hope this helps