The enderdragon sometimes flies too far and despawns untill you come to it. When it does despawn, its health bar also disappears

Figured out the chunkloading thing, the dragon will keep 3x3 chunk area alive until you kill it.

It doesn't despawn, it gets unloaded; it's really annoying, but I don't know what I could do with it, apart from making it an entity chunkloader, for which I need some documentation because I've no idea what I've done wrong the last time I tried it.