After setting up spawn protection I deoped myself and tried to break blocks at spawn. Blocks shouldn't break, but they did: particles showed up an sounds were played, but blocks were immediately placed back by server.

Also, after placing a block it appeared for a moment and then was replaced by air.

I don't know if it's a bug or a intended mechanic, but I decided to report it.

You can place block inside of protected area by placing them on the side of block outside of spawn.