I have brought back many dolphin's to my base (Around 400 blocks from their spawn ocean) only to have them Despawn. Please note, I said despawn, not die.

First off - let me list my fixes that I've attempted and that will explain that this is indeed a bug.

• I have had nametagged - from a named Egg and a nametag - despawn. But, this isn't MC-27535 as unnamed dolphins also despawn.

• I have had them on a lead (within spawn chunks, so the lead shouldn't unload) pulling them up for air - but they despawn.

• I have gave them items to play with and had them trust me by feeding them raw cod - despawned.

• Eventually I realized that the AI is probably a mess, so they will drown. So I made the water only 1 block deep, and had a redstone circuit power pistons to repeatedly push it out into the air. Still despawned.

• This is when I started to get annoyed. So, I decided to set myself into spectator - boom, they despawned (though I haven't had this happen again.)

• I then realized every time I went into the nether, they despawned.

• Anytime I teleport far enough away to unload the chunks, they despawn (Even though my enclosure is within the spawn chunks.)

• Heck, I have even been in my house, within 20 blocks of the dolphins that are nametagged, on a lead, and on my piston machine despawn on me.

• I then found the issue of mobs despawning when on chunk boundaries, but sure enough, the enclosure is in the middle of a chunk.

• I finally decided to put hoppers underneath my enclosure to see if they were actually dying and dropping anything, or just despawning. My chests have collected nothing, so, they are not dying.

HOW TO REPRODUCE: I don't have any exact steps, but just go into a creative world, spawn a named Dolphin, turn your back and/or go away - and chances are you will find it missing, having dropped nothing (more proof that it is despawning and not dying.)

I have had a dolphin last a few real world days and I have had dolphins last a whole of 30 seconds. It seems very random. Granted, I do have loads of mobs in my house (as my goal was to collect every single animal in the game) but this isn't some server overhead issue where it's deleting mobs - as if it were, I would notice one of my 15 horses, 20 tropical fish, 30+ pandas, 4 llamas, golems, 7 rabbits, 11 cats, 8 dogs, etc, despawning as well. Yet time and time again, it's only the dolphin. Heck, even my nametagged squid is still around.

EDIT: Exact steps to reproduce - Go into creative, build an enclosure within the spawn chunks (or build an enclosure right by a naturally spawned dolphin, so close to it's natural spawn point), name the dolphin with a nametag OR use a named spawn egg, Teleport far enough away to unload the chunks (I just kept using 8000x and 8000z) and teleport back to the dolphins. You will find they dissapeared. Do it quick enough and there is no way that they 'die' from suffocation or drowning, as it takes 2 minutes outside of the water and 2 minutes inside the water to start taking damage.

EDIT 2: Just added a video demonstrating how to reproduce. Hopefully this is confirmed soon. You can try this with other mobs that typically despawn (Stray cats, Ocelots, Squids) and if they're nametagged, they will still be there when you teleport back. Hopefully this gets confirmed soon.