A fairly serious bug for survival play. I made a report on this before, however, it was assumed to have been directly related to an issue in Realms, which I have confirmed in video this time to be untrue. I have upgraded from playing on Android to playing on Windows 10, and can 100% confirm the issue to be cross platform, and not isolated to realms or to be device specific.

it should be noted that I had done nothing to the dragon prior to this bug occurring. I had destroyed a single end crystal without the dragon being healed by it, however, this was not shown in the video that will be linked below. I would upload the video in here, however, doing so through YouTube will be easier, and will allow for this bug to gain some publicity so others may find it and aid in reproducing it.

It should also be noted that /kill @e does NOT work on the dragon when it is in this immune state, however arrows still damage the dragon without issue. I have not tested this with tridents.

Also, the only add-ons I had was a custom music pack, and a resource pack I had created to replace the pumpkin blur with an icon to make it less obnoxious to wear in the end

Prior to recording, the bug had already affected the dragon. So killing the crystals with commands did not have any bearing on the state of the dragon.

These are all the details I've touched upon in the video. And please, don't make me have to re upload the video here. It's a large video file, and with a weak internet connection with an upload speed of only 400kb, I'd have to sit here for another 2 hours waiting for it to upload.

Small update before I post this: I did do one other thing in the end. I had flown away from the end dragon to remove the boss bar and allow the default end soundtrack to play. I'm not entirely sure if this is the cause of the bug, but if so, it could easily be an issue with the game trying to despawn the dragon, but not doing so properly. Which could make it related to a very old bug where the end dragon did used to despawn and cause the exit portal to open, but was patched in (I believe) 1.7

Update #2: Upon exiting the end and reentering it, the dragon was once again attackable via melee, and the bug did not reoccur when flying away from the central island and back. Will be performing further testing after posting this bug. Yes, I'm marking updates this way as they're after the video.

https://youtu.be/bp84EeWgjVM - the video of the bug