Sounds like all the new unprogrammed Autel MX sensors you bought came out of box read FFFFFFF before they have a unique ID programmed to them.
It still doesn't answer the question that once an ID is programmed to a particular Autel MXsensor, can the seven digit hex ID wrote to the Autel MXsensor ever be edited or changed to a different ID you prefer it to have? Example If you make a mistake and write a redundant ID to one and have to change to correct ID , or I buy a used wheel for my car that has a used Autel MX Sensor that has previously had an ID wrote to it. Can the ID be overwritten with another ID already in the car module I would prefer it would have, so I don't need to go into the TPMS module with scanner and write in a different ID to the wheel position on the car or have to do a relearn procedure?