I don't know where your block heater is installed, but is it possible that the heater is warming up the temp sensor for the fuel injection and when the car starts it thinks it is warmer than it is. Once the water circulates a little the sensor realizes how cold it is and compensates with more fuel ?
Going farther out on a limb, when you unplug the heater and wait 15 minutes, the plume of warm water thermosiphons past the sensor and pulls some cooler water in behind it. At that point it acts normally.
Greg
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