Usually killing the dragon produces a levitation animation and it begins to disappear, however in the latest beta/preview versions the animation of levitating after dying is not played.

In the video the dragon reaches the exit portal, but even if it dies in the exit portal or in the air, there is no levitation in the animation and the dragon remains frozen at its point of death

In the following videos we can see the bug in the video with the title "EnderDragon_Death_bug", while in the "EnderDragon_Death_No_Bug" It is the video that shows what the correct animation should be like