AlsEU is right, one (or more) of the printheads providing wrong temperature values. But the temperature value shown in the GUI is just an visualization of the average of all temperature sensors together. This value is not used to activate or deactivate the heater system. The heating system is self-controlled with temperature sensors on ink reservoir in carriage and coolant reservoir in right side gantry. The only sensor that deactivate the heating is „coolant low“-sensor.
Print starts if temperature measured in one slot (default 5, first cyan) reached the threshold. The default can be changed in registry.
Does the coolant pump and degassing pump start to work on heating? Does the heater elements on ink an coolant reservoir start to heat?