I am considering the purchase of an Ultra 2 Watch. The ones on the Apple Store make it clear the sensor is disabled. I am wondering if the Ultra 2 watches from other vendors might not be disabled? Is the disabling happening when the watch is activated? or is it happening based upon the date it was assembled? I really don't want a crippled watch as some of the fitness features that rely on that sensor are important to me. Thanks for any advice?