Ok I figured it out!
It was an issue with the WP-Property plugin that I’m using.
There is an option for this plugin to set the “Default Properties Page”. You can set this option to one of your static pages, or use the default option which is internal to the plugin.
I had it set to use the Parent Page of the 3 pages I was having the issue with. For some reason any child pages are set as post when this option is set to anything other than the default.