It might have been routed at that altitude to deconflict with other aircraft.
AF1 and other aircraft carrying heads of state usually have a security vertical altitude & lateral distance 'bubble' around them that other aircraft are not permitted to enter.
With AF1 & other high value targets they might've tweeked the ADSB tracker system to give a false height or position for security reasons.
Potus tweeted recently about having 'problems' with ADSB tracker recording height & other details correctly.