Create collection takes a database exclusive lock and then checks the FCV to determine whether to create a collection with a UUID or not. The setFeatureCompatibilityVersion command, however, sets the FCV to upgrading without taking any locks. Therefore, createCollection could get as far as checking the FCV, not seeing upgradingTo36 or fullyUpgradedTo36, then the setFeatureCompatibilityVersion can assign all the existing collections to have UUIDs, and finally the createCollection finishes without assigning a UUID to the new collection.