Same issue, The mobs that are telported are not interactabl and get vanishes after you quit and join again.
Suppose you are at place 'A',then the mobs at this place will be visible but not interactable at base 'B'.
Base 'A' and 'B' can be millions of blocks away or just 500 blocks away.
No. I play on single player and on peaceful difficulty most of the times
So I am not also sure that active mobs like tridnet holding mobs also spawning in air or not.
I observed that the fish are spawning at y level 33.