Is it ok for a water heater pressure relief valve to be moved away from the tank with a short pipe?
What is your water pressure? Do you have an expansion tank? Lack of a (working) expansion tank is the most common cause of overpressure releases (meaning the valve is doing what it's supposed to do, not malfunctioning.) Heated water is physically larger than unheated water, and pressure can rise due to that if no water is being used.
Extension makes no difference to pressure function, may or may not affect temperature operation (depends on probe length of the new valve, which might not have been the same as the original valve.)