Leaving world when riding stacked entities unstacks or deletes the stack

A player on a pig in a minecart who relogs will cause the minecart to disappear.
See this video by sethbling.

Alternatively, see this video to see what it looks like from another player's perspective.

Steps to reproduce:
1. Run this command to create the needed entities:

/summon Pig ~ ~5 ~ {Saddle:1,Riding:{id:Pig,Riding:{id:Pig}}}

2. Mount uppermost entity
3. Disconnect/Unload world
4. Join/Load world

Result:
Local game: Deletes all but the uppermost entity of the stack
External Server: Decouples the uppermost entity from the stack; all other entities remain stacked correctly.