Prefixing the default post permalink and setting page permalink to the same value doesn't work.

Seems to only affect the local jekyll serve WEBrick server. GitHub Pages, for example, isn't affected.

• I believe this to be a bug, not a question about using Jekyll.
• I updated to the latest Jekyll (or) if on GitHub Pages to the latest github-pages
• I ran jekyll doctor to check my configuration
• I read the CONTRIBUTION file at https://jekyllrb.com/docs/contributing/
• This is a feature request.

• I am on (or have tested on) macOS 10+
• I am on (or have tested on) Debian/Ubuntu GNU/Linux
• I am on (or have tested on) Fedora GNU/Linux
• I am on (or have tested on) Arch GNU/Linux
• I am on (or have tested on) Other GNU/Linux
• I am on (or have tested on) Windows 10+

• I was trying to install.
• There is a broken Plugin API.
• I had an error on GitHub Pages, and I have reproduced it locally.
• I had an error on GitHub Pages, and GitHub Support said it was a Jekyll Bug.
• I had an error on GitHub Pages and I did not test it locally.
• I was trying to build.
• It was another bug.

My Reproduction Steps

• in _config.yml, set permalink to /blog/:title
• create a page and set its permalink to /blog
• run jekyll serve
• visit localhost:4000/blog
  • Note that you are seeing the WEBrick server's directory index, not your page
• visit localhost:4000/blog.html
  • Note that the page does exist
• in _config.yml, set permalink to /notblog/:title
• rerun jekyll serve
• visit localhost:4000/blog
  • Note that it works correctly now

The Output I Wanted

I expected that visiting the permalink defined in the page's front matter would display that page.

Related to #3452