You're only clearing the value when there isn't a new value. When the field is erased there is still a value there. Make the following changes to your code.

if (isLoading) {
  return;
}

AND 

if (newValue == ""){

g_form.clearValue('u_advisor_number');
g_form.clearValue('u_advisor_region');
g_form.clearValue('u_advisor_district');
g_form.clearValue('u_district_manager');

}

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