thats strange. if you short the TEC, it should be actively trying to equalize the temperature on both sides, the counter effect is the flowing current.
Shorting the TEC should lower the temperature resistance between the faces.
With a high load the current cant flow, and the temperature has to equalize by normal means
Either something is off, or i totally miss understand something :/
edit:
on second thought. i THINK, your statement cant be right. If i put a fixed amount of energy on one side, i can use this energy to create a current across a resistor. The heat will be transferred equally to the other side. If i increase the electric load, more current will flow. The heat will then faster equalize. If that is not the case it would imply i can suck more energy out of the heat if i increase the load, which cant be right.