How to get WordPress to NOT alter the permalink structure when adding a static page?

It sounds like user error to me, or possibly a plug-in. I am leaning towards operator error. WordPress just gives you the vehicle, your the one at the wheel.

The good thing about WordPress is it allows people without the knowledge it takes to build a website a chance to jump right in, sink or swim. It’s all a matter of how you set it up. You should always plan ahead and think about how your site will be set up in relation to “Front Page”, “Posts” or Blog, categories etc…

If you changed your site from front page displays: a Static Page, then switched it, you might have some “hard-coded” links that are not re-directing. Have you tried visiting the Permalinks page and clicking the update button to see if it fixes it?