The bug

When you are riding an entity and teleporting through portals, you and the entity may teleport to wrong place and fall/suffocate/fall into the void.

Steps to reproduce:

1.Create a superflat world (This bug can happen in default world type, but in superflat worlds it seems always occurs)

2./gamerule entitiesWithPassengersCanUsePortals true

3.Go into the nether.

4.Summon and tame a horse and ride it.

5.Go back from the nether to overworld with it through the portal.

Observed result:

You are teleported into mid-air and fall, then the horse died(in survival you will also die)

Expected result:

You are teleported normally to the overworld portal.

Notes:

This affects both nether portals and end portals.