When you move the Spawnpoint with the /setworldspawn and then go in to the Nether and exit it again, the spawnpoint gets set to 0,0. It happens on servers and in singleplayer. If you disconnect and join a server it is not happening.

Also happens when you just create a new World and go to the nether and back. The spawnpoint will then be changed to 0,0.

Happens also in 15w36d.