When entities other than players (mobs, falling blocks, items, etc) come through the exit portal, they do enter the overworld at world spawn, but at Y level 32767. 

This screenshot was taken with another player loading the exit portal chunks and a command block summoning primed tnt above the portal

This issue was first encountered on a realm with players from windows, ps4 and xbox one.

This was recreated for testing in a standard multiplayer world with players from windows and ps4