I have run into three occurrences in which an Ender Dragon has spawned while I was transiting a portal.

First occurrence- teleported through a end portal there was a "thunk" noise and I was flung into the void and died.

Second occurrence- teleported through the same portal but this time I was wearing Elytra, had rockets in my hot bar, and drank a potion of slow falling just prior to teleporting. Again heard the "thunk" noise but remained standing on a portion of the bedrock surrounding the portal and saw a dragon flying away. Tried flying after the dragon but it quickly flew away and disappeared.

Assume the "thunk" noise was caused by the dragon's wings and they knocked me into the void the first time and would have the second time if I hadn't been saved by the potion.

Third occurrence- was transiting from the overworld to the end using the portal in the fortress. Had successfully made transit and moved from the portal to my end base, but 2-3 seconds later the ender dragon appeared from the portal, followed a path to me in the base end base and destroyed many chests when it flew through the blocks, erased portions of the garden, and killed all my chickens, I prepared for battle after clearing up the items from the chests. I could hear the dragon flying around and thought that some how it had been summoned, but when I checked all the healing towers were still broken. There was no health bar for the dragon and even though I could shoot arrows and get a red flash when they hit, I had no way to observe total damage. I returned to my end base it get better equipped but suddenly the dragon just vanished, No display of any kind just gone.

Not sure what is going on but don't think this is normal dragon behavior