Your boiler is switching from controlling the water temperature based on the outgoing flow to the returning water because it has detected a more stable reading that way.

What causes this fault?

This occurs when the boiler detects that the temperature sensor on the return pipe is providing a more reliable reading than the one on the flow pipe. It physically happens because the control board senses a conflict or inconsistency in how heat is being measured as water leaves the unit, prompting it to switch its monitoring method to ensure safe operation. The most common reasons are a drifting calibration in a sensor or a slight restriction in water flow through the heat exchanger.