Your boiler is still detecting a flame for a short time after it has been told to turn off, which is a safety concern that requires an engineer.

Flame rectification present 4 secs. after gas valve turns off | Check gas valve, burner tubes and injectors or electronics defect

What causes this fault?

This fault occurs when the boiler's control system detects that gas is still flowing into the burner even after it has commanded the unit to shut down. The most common reason is a mechanical failure where the internal seals of the gas valve become stuck open or the electronic circuit board incorrectly sends power to the valve. Because this represents a significant safety risk, the boiler will immediately stop operating to prevent an uncontrolled flame.