Summary:
Mobs that go through the exit end portal dissappear and will only appear again if the player leaves then re-joins the world.
After re-joining the world, all the mobs (including items) that went through the portal, spawn at the wrong height and concequently will die from fall damage. Items will behave differently, they will fall from the sky in random locations, but will not be lost. This issue doesn't affect the player's positioning, nor does it inflict damage.

Steps to reproduce:

1. Push a mob or drop an item into the exit end portal.
2. Jump into the portal to get back to the overworld.
3. Leave then re-join the world.

Observed results:

Prior to leaving then re-joining the world, all mobs and items are seemingly gone, after leaving then re-joining all mobs will die from fall damage and items will spread out in random locations (won't stray too far from the world spawn though).

Expected results:
Mobs and items should appear normally atop of the world spawn blocks.

Workaround:

Placing water in the world spawn location will prevent mobs form taking fall damage.

Links:

• Glitch showcase video: https://1drv.ms/v/s!AlHTNYryikxli36HJFHTfRdj0JuS?e=aQkqOh
• World file (this world was created in the year 2020, on the 1.16 version): https://1drv.ms/u/s!AlHTNYryikxli3-AvaJxcnPo1o4_?e=9pDg0w