John in NM
If you can, hardwire the heaters in with screw terminals. Lee is correct about outlet quality, but also the nature of outlet connections makes them the high impedance point in a circuit. We normally don't notice, because our circuits are typically lightly loaded and loaded at a low duty cycle. Running a heater or two all night is a heavy load at a high duty cycle.
On the other hand, the good thing about how it's tricking the thermostat is that this is reducing your duty cycle somewhat and therefore reducing the risk of a fire.