I think a little background is needed for both of you fine anons. These sites track from two types of feeds generally - FAA (radar + multilateration, delayed 5min) and ADS-B/MLAT (multilateration). ADS-B is the “new Radar” that all commercial/private flights have been mandated by the FAA to have installed by like 2020. ADS-B is different than radar because the aircraft reports its position (gps), speed, and altitude (plus flight info). Radar requires being hit with radar waves (terrain/obstructions can be an issue) and you dont get flight data.
Military flights don’t use ads-b widely. They can turn it on/off, reprogram flight data/callsigns, etc. When’s visibility isn’t concern and operating in civilian airspace, they tend to use ADS-B.
Commercial flights with ads-b always have it running. Civilian flights with ads-b tend to leave it on as well (investment in hw, safety reasons).
MLAT works off of the planes transponder. Flight data is transmitted, three or more MLAT stations are needed to triangulate position.
ADS-B/MLAT data comes mostly from rtl_sdr, radio, or aviation enthusiasts (which feed data to flight tracking sites).
ADS-B (and MLAT largely) are line of sight. Running one personally here - anything that is too far or two low will drop off the map suddenly.
So if a plane disappears, it could be as simple as the aircraft isn’t within range of an ads-b site. I think it’s less likely are that ads-b was turned off or the flight was brought down.