Minecraft – My minecraft world spawn keeps setting underground

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!

Reduce the spawn area to as small as you can, and then remove all land around it.

This way, cheaters have nothing to break when they spawn -- it's just air.

If you can get the command blocks to work remotely (which I think you are, if I read your post right), then they won't be able to break the command blocks either.

Alternatively, you could do this with Bedrock.

As a side note, especially if this doesn't work for you, you may want to change the title of this post to a question. The current one doesn't tell anyone what your actual question is.