WorldSpawn Bug

I've set my spawn in a sign with the command:

/spawnpoint

After that, I killed myself.

What I aspected:
My spawn gets "destroyed" and I will respawn in the spawnarea with the worldspawn. Also the chat would say: "Your home bed was missing or obstructed"

What happened:

I spawned exakt on the worldspawn, even when there where blocks above me (like Commandblocks and Blocks of Quartz)

PS: I've set the worldspawn with the command:

/setworldspawn 0 0 0

Later, I respawned at 0 0 0