Ok, I tested again. As soon as the Gainful pops up in my HAS FoV, I lock, wait for RDY label and launch. Missed missile. But, if I wait until the Gainful appears in my RWR, then bang! HARM magnificently obliterate the radar.
May the bug be that radars appear in the HAS view even when they are not working or out of range? Because using HARM as Sensor, I can suppose that if I have a steady lock before launch, it will be the same after.
Regards!