Falling on entering a world

I'm sure this bug is here somewhere, but I haven't found it.

In the latest snapshots I'm having the problem when logging in sometimes I spawn falling through the world. If I spot this soon enough I can quit and login again and all is well.

The trouble is that I'm getting this after I jump in the end portal and spawn on the obsidian platform in the end. This happens about 1 in 10 times? Quitting and logging in again doesn't work around this bug, I just die (and lose all my stuff!)