No heat on high RPM

A driver complained to me that his truck (07 Volvo VN Cummins) blows hot air as soon as he gets on the highway. I replaced the HVAC control panel, checked fan clutch, replaced heater core valve, checked all the air vents actuators. Everything worked perfect on idle. I took it for a test drive on a highway. I switched it to cold (his AC is not working, so he switched it to cold to defog the windows). When I switched it back to hot, it stayed cold (the fan works, just blows cold air). I couldn't see if the air duct closed or not. But as soon as I switched to neutral to take the exit in a matter of 2-3 seconds it started blowing hot again. Same happened later when I simply pressed the clutch for a few seconds on the next try. So it looks like it has something to do with the RPM. How can two be connected? Can ECM be the culprit?

PS I showed the driver the trick to get the heat back and he left. But then he called me and complained that heat disappears when he hits above 1500RPM (70mph at 10-speed, I'm not sure I reached it when I was test-driving it) even if he didn't switch it to cold and reappears when RPMs drop. But this is un-confirmed by me as of now..