[KarlInOhio]

and then fell off radar as they approached Iran.

These trackers are probably reporting transponder information rather that the primary radar returns, so the Chinese pilots are turning off their transponders when they get close to Iran.

[Drago]

“FlightRadar24” and similar websites like “FlightAware” use “ADS-B” data not radar or transponder data (transponder data/interrogation is a subset of radar signals). The military routinely turns off their ADS-B if on a mission (and if equipped, a lot of military aircraft are not equipped with ADS-B yet). Chinese civilian 747’s could turn off their ADS-B, but the FAA in US airspace doesn’t like that.

https://www.flightradar24.com/blog/most-wanted-receiver-locations/

https://en.wikipedia.org/wiki/Automatic_Dependent_Surveillance–Broadcast