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Oh, I can tell you exactly where I have documented this: https://www.akeeba.com/documentation/akeeba-backup-joomla/restoring-backups.html#general-guidelines under “Do not restore in a subdirectory of your main site.” Let me copy & paste the pertinent two paragraphs (they apply for new site installations as well as restored sites): 

Do not restore in a subdirectory of your main site. For example, if your site's root is in public_html do not restore to public_html/dev. The reason is that the .htaccessfiles, which tell Apache (your web server) how to server your site, cascade. That is, Apache will read all .htaccess files in all folders leading to the one hosting your site'sindex.php file. This will cause problems with the restored site which you will experience as 404, 403 and 500 error messages or blank pages. These have nothing to do with our software and / or the restoration. It's how your web server works. Use a subdomain instead.

If you are restoring on a subdomain, make sure that the subdomain's root directory is NOT a subdirectory of your main site. This is the same as the previous paragraph, really. Most hosting control panel software default to using a subdirectory of your site's root when creating a subdomain. For example, if your site is www.example.com and its root is public_html if you create the subdomain dev.example.com your hosting control panel will put its root in public_html/dev. Therefore you will have the problem we described above. In this case ask your host what is the best way to create a root folder for the subdomain next to public_html, not inside it.