When a new player joins my server, all players previously connected become invisible. The new player is visible, however the new player cannot see anyone else.

If an existing player disconnects, and re-connects, the new player can see them, however the existing player cannot see anyone else.

Animals, and frames are usually visible during this time.

The new player can be attacked by existing players, they take damage, and the new player's client shows them being attacked, by taking damage, and some of the swhoosh graphic swords create now can be seen, but no sword, or attacking player.

New player can not see existing player, however if new player is positioned in front of an existing player and swings at existing player, existing player does not take damage.

New player cannot interact with existing players other than via chat. There is no hit box, and there is no collision detection by either the new or existing player while on the server.

Also appears to be experienced by other players:

https://www.reddit.com/r/Minecraft/comments/3keoyh/minecraft_snapshot_15w37a/cux10s3