It seems to happen when a president leaves US territory. It goes back to the tail number when in country at least on adsb sites; as we all know whatever plane the president is on becomes Air Force 1 regardless of whatever information gets punched into the adsb radio. This adsb behaviour happened as far back as obama maybe longer.

It was also very public when Trump was doing his rounds on the compaign trail as people tracked his flight into each airport it woudl show the tail number of the plane. Only showing "AF1" when he was out of country.