Might be a defect mosfet on your board. From pic I see firmware tells it to not heat but temp. rises smoothly so reading is ok but it heats without being enabled.
So you have either a short in sending pins and an other pin also can heat the mosfet or it is just defect and sometimes decides regardless of input. Try the other mosfet for extruder and see if it also happens there. Be aware the D9 is also fan output so don't enable fan feature or select other pin for fan.
Alternative reason would be an other function uses the pin as well e.g. fan and when fan goes 100% it enables heater instead. But that normally gives problems during heating at the beginning whcih you do not have, so I don't thinkit is the problem.
