Why isn't spawn-protection protecting the spawn?

I've run into a headache setting up a Minecraft server (using the FTB modpack). Having set up a sweet open air spawn-jail and moved the spawn point into it (using MCedit), I started hitting things as a non-op and noticed that the default spawn protection of 16 blocks radius worked fine for inside the house but wasn't enough to keep people from messing up the lawn, so I set it to 42 blocks in the server.properties file.

Now spawn-protection seems to be entirely disabled for non-op players, which I thought wasn't possible! Changing it back to spawn-protection=16 doesn't restore spawn protection either. Why isn't spawn-protection working on my server?

Details and proactively answering likely questions:

• The spawn protection was absolutely working before I messed with the server.properties file.
• I have restarted the server every time I've changed the file.
• I have tried breaking blocks both with an account that was opped + Creative Mode before but deopped + Survival Mode to test, as well as with another account that was never opped or switched from Survival.
• The spawn point is legal according to MCedit.
• Running Feed The Beast BETA Modpack A v12 (which means Minecraft 1.4.2)

• The spawn jail looks like this (the spawn point is in the middle of the third floor, which is the top row of windows):

  

• My server.properties file:

  #Minecraft server properties
  #Wed Dec 19 12:39:24 PST 2012
  generator-settings=
  allow-nether=true
  level-name=FTB-Beta-A
  enable-query=false
  allow-flight=false
  server-port=25565
  level-type=DEFAULT
  enable-rcon=false
  level-seed=
  server-ip=
  max-build-height=256
  spawn-npcs=true
  white-list=false
  spawn-animals=true
  snooper-enabled=true
  hardcore=false
  texture-pack=
  online-mode=true
  pvp=false
  difficulty=3
  gamemode=0
  max-players=25
  spawn-monsters=true
  generate-structures=true
  view-distance=10
  spawn-protection=16
  motd=Minecraft With Friends
  

Solved it!

A meandering discussion on the Talk page for the server.properties article on the Minecraft Wiki indicates that this is Forge's fault. Apparently it redefines spawn-protection to be a boolean that turns protection on or off entirely (which you can't do in vanilla), and defines a new variable, spawn-protection-size for what the former variable means for a vanilla server.

As a side effect, it appears that a server with the Forge API installed will honour the default spawn-protection area of 16 blocks until you edit the variable, at which point it treats it as boolean and looks for spawn-protection-size instead, doesn't find it, and effectively turns spawn protection off entirely.

So this is what happened to me then. Adding spawn-protection-size=42 to my server.properties file made spawn protection work (again) as expected. Redefining configuration variables is poor practice!