That means the transponder became non-viable at that point. Any sort of in-air catastrophe could have caused that, including onboard fire.
Redundant airplane systems protect against component failure, but they tend to still be near each other. For example, if an AA gun were to tag the plane in the right place it could easily take out power to both a primary and backup.
It really doesn't look like a mid-air explosion. It was either a massive spontaneous fire for some reason (assuming the burning thing in the social media post was the plane) or it got shot down.
That doesn't mean it exploded mid-air. If it exploded mid-air where the FlightRadar24 data end, the debris field would be in the direction of the flight and spread wide. Debris field is in the opposite direction and elsewhere though.
Also the way the debris were scattered on the ground suggests an explosion. This also correlates with what they said about no mayday call from the aircraft. It was taken down or it was a collission with a drone or something like that.
Lemme be blunt. Some people believe that since this happened at a time military tensions between Iran and the US were escalating, the plane could have been taken down mistakenly by a trigger-happy general as a false positive.
Perhaps. But a general of which side? It was, I think, over Iran, so Iran's air defense would be the logical suspect. And Iran withholding the flight recorders would fit with that.
The more paranoid, of course, will look beyond the logical suspects...
Last time a civilian aircraft was shot down over Iranian airspace it was the US. And while they agreed to some “no-blame” deal including a financial compensation they never actually apologized for murdering 290 innocent people. https://en.m.wikipedia.org/wiki/Iran_Air_Flight_655