Items go missing when changing Versions

This happens in Creative and Survival modes.
using the new launcher, I started Release 1.6.1 and opened an existing saved map. I explored and found a horse and tied it up to a rail.
I exited and restarted in 1.5.2, and as expected, the horse was now gone.
I then exited again and went back into 1.6.1, but the horse was still gone.
I expected that the game would save data for maps based on the version launched. So that going between game version would load a map's data for that version.