I cannot describe this bug fully because i'm tired right now, so i recommend you to watch this video: PHANTOM NETHER GLITCH in MINECRAFT?!

But if you don't want to watch the video, i'll tell you (feeling forced) about the details of this bug (based on that video, okay!). This bug involves entering the Nether via the End.

• Terrain generates as normal, but it doesn't spawn mobs at all, even in the difficulties higher than Peaceful and even in Survival mode!
• No Blazes spawn from their spawners.
• Entities will generate only when you enter the Survival Mode. In Creative they don't.
• Enchanting Table's book stays in place and doesn't move.
• Those entities, particularly mobs like Zombie Pigman or Blazes if summoned (or if they are already generated before) doesn't have an active AI and cannot be interacted except by hitting them. However, they doesn't react (stays in place) and after that can't be interacted again. I don't really know if they die, but i expect they will stand still instead of leaning towards the ground and after that they despawn (they will not drop items at all).
• Sand or gravel (or all other blocks that can fall) won't fall in Creative Mode but they do in Survival mode, but they will "fall infinitely": once they reach the ground, they won't instantly become a block and instead afterwards they will return to their original position and they'll fall again. You can do this by putting a sand or gravel or all other blocks that can fall via Creative Mode (An air block should be exist below it) and then change it into Survival Mode so you can see them falling and repeating it again.
• If you shoot arrows from the bow, they won't show up in Creative Mode but they do in Survival Mode in where you fired them on, and afterwards however they will stick on the air where you fired them on.
• Blocks cannot be broken in Survival Mode, but they do in Creative Mode.
• Based on it, if you break the block in Creative or "breaking" it in Survival Mode, arrows that is stuck on air will fall below their position they originally stuck in, then return back.
• Dropped item entities won't appear in Creative mode and if you change it to Survival they do, but they are unacquirable and they'll stuck in "fall infinitely" state like the falling block entities before.
• Redstone does work fine but some of them particularly pertaining to entities does not. For example moving piston heads, droppers, dispensers, minecarts, etc.
• Beds do explode normally, but the explosion particles doesn't appear and some of the blocks that are supposed to drop won't appear (in Creative) but as expected they do in Survival but they, again aren't acquirable and stuck in "fall infinitely" state as expected.
• If TNT is activated on Creative mode they'll disappear (Since Primed TNT is an entity), but if in Survival they will not disappear as expected. However, they don't explode after a while.

These effects are will be disappeared and will return to normal if you quit the world then return back. However they can be returned by going to the End and then enter Nether again.

This bug is discovered by xDarkDiamond, then showed off to the public by AntVenom then afterwards i reported this to the Mojang, in hopes that this bug will be eliminated.

Reproduce:

• Use this command first: /setblock ~ ~ ~ minecraft:end_portal
• Then use this: /setblock ~ ~ ~ portal
• Try to do some of them above like shooting arrows with a bow in there or watch the video first, then do the activities that AntVenom does. (if you want to do it better)
• You'll found some of the problems i have shown it above.

I'm sorry for not giving the screenshots because my computer's graphics card is broken, but please don't worry because you can see the video for it's effects and i'll end here for now.

Anyways, thanks!