I think torque is more at cause here. The a/c gives resistance to the motor, and the engine translates the power in torque to counter that resistance. Past a certain RPM, torque is sufficiently high to free up the power of the engine and drive the a/c comfortably. But mileage will suffer a bit.
This will not answer the question though, but it would not surprise me if the a/c was not turned off.
The M50 is pretty slow to start in low revs because the torque is too low there. That engine really wakes up above 3000 rpms, I find.