End Dragon is glitched, will only spawn for a few minutes then disappear. We have two dragon's nests. One will always have the portal active, even if the dragon is attacking. Normally we will summon the dragon, it will spawn, sometimes create new towers. It will attack a few times then fly off. 

One time we found him glitched in the overworld at the spawn point while we were righting a wither. He was flapping his wings in the ground, and was not attackable. Another time he was doing the same thing in the end, then disappeared. This is on a realm and happens no matter how we spawn him, in peaceful or with monsters turned on. This is a survival server.