In an odd situation, I have a site running Drupal 7 installed in the root directory. I want to install a Drupal 8 site into a subdirectory – but when I go to www.example.com/drupal-8/ I get redirected to the Drupal 7 page not found. Worth noting is that there is another Drupal 7 install in another subdirectory and that site can be accessed via www.example.com/drupal-7/. I imagine it may have something to do with .htaccess but have been unsuccessful thus far.
Most of what I have been able to find discuss subdirectory installs where they want to mask the subdirectory path OR discuss issues related to multi-site installations.