Upon entering the nether from the end, entities render invisible and unusable when placed.

In the end, summon a nether portal block and walk through. Upon entering the Nether, things should appear to generate normally. However, try to place an item frame on a block, and it will not display or be usable (blocks cannot be placed in invisible frame or will not display). The same issue occurs with armor stands. After placement, these invisible entities are invincible. Switching from creative mode to any other mode (or vice versa) will make the entities visible but they still cannot be broken, and will even float in air. 

Testing with spawn eggs, the mob sounds as if it has spawned but is invisible and doesn't move. Upon switching gamemodes the mob becomes visible but does not move. It can be hit once (does not change position) but afterward appears invincible and will not move even if floating in air.

Going to the overworld and then returning to the nether renders all such entities functional and able to be harmed/interacted with, and entities can be placed normally from this state.

Pictures depict custom built nether portal (summoned piece by piece with /setblock in the end), and an assortment of visible, invincible mobs and entities floating in midair. View of invisible entities not included because, frankly, there was nothing to see, but entities did not appear until change in gamemode. Additional image showing the updated states of the entities after leaving and returning via overworld to nether.