Happens all the time. One of two things going on here. First it's important to understand that ADS-Bx Network is not some all seeing eye in the sky that will track all aircraft wherever they may go. There has to be a network member receiver within range to pick up the signal. And when aircraft fly out to sea they will drop off coverage. And the network will stop updating them until they fly back into range of a network receiver. See coverage map image attached. Secondly, when a US Government VIP aircraft flies out over international waters they will usually swap hex ids and call signs for reasons of security. Many times, not always, you can see one aircraft stop and another one just pop up at this point. Both outbound and inbound CONUS.
https://www.adsbexchange.com/coverage-map/
https://www.adsbexchange.com/wp-content/uploads/MicrosoftTeams-image-7.jpg