Automatic Account Creation will only work on the very first setup of a device. Setting up a device after this, even if you have re-installed the OS, will require a manual account creation.
This is because Fleetsmith does not recognize a re-provisioned device as a "Pending Device" until it has already accessed the Setup Assistant. Therefore, the device can't be approved and assigned before going through the Setup Assistant.
While it is technically possible for Automatic Account Creation to happen if the device receives the MDM command from Fleetsmith at precisely the right time, the workflow to achieve this is complex and far too prone to error. Until we're able to build a workflow which would make this process less brittle and labor intensive for our customers, Automatic Account Creation is not supported on re-provisioned devices.