AFAIK Flightradar and some others provide a VIP service for blocking flight data. Elon's Jet is blocked there, too. Ironically, the owner of the elonjet account even told Elon how he could hide flight information on these platforms...

> Last week, Sweeney noticed he couldn't locate Musk's jet, and realized Musk was attempting to hide his flight patterns from the @ElonJet using the blocking software Sweeney had recommended to him.

Source: https://techxplore.com/news/2022-05-college-student-tracks-e...

It's very likely filtered out of the FR24 data. Use https://globe.adsbexchange.com instead and you will get everything, including Elon's jet.