The problem is it is not spawning you within 20 blocks, it ends up being in the hundreds. Like 200 blocks away. Here is an example of the difference I experienced. Spawn point set to -159 70 479, final resting spot -87 73 364. Alot more than a few blocks. When I created a fresh server world with default server properties, it works just fine. Only with this setup does it fail. The only thing I can think is the spawn protection setting. I have it set to 150 instead of 10. But I cant imagine that would effect were people spawn. Or at least it should not.
MC-62003
server side world spawn fails with custom server properties
Resolved
Invalid
0
The user's spawn range is spawn protection area minus13 blocks.
With your setup, the players can spawn in an area of +- 137 blocks around the spawn point.
See http://minecraft.gamepedia.com/Spawn/Multiplayer_Details and http://minecraft.gamepedia.com/Server.properties