Ignore mod_rewrite redirect for subfolder with AuthType Basic

I had a few admin PHP pages in a custom subfolder for a customer, protected with the AuthUser directive in the .htaccess file.  However, when I migrated their main site from Zen Cart to Magento (moving the Magento folder to the root), the subfolder kept throwing 404 errors from Magento.

I knew which line was doing it, but the final redirect should have been ignoring real files and folders:

RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteCond %{REQUEST_FILENAME} !-l
RewriteRule .* index.php [L]

Finally I stumbled across the answer, which was not on the main .htaccess folder. In the .htaccess file in the protected subfolder, I needed to add a single line, which probably should have been there the whole time:

ErrorDocument 401 "Unauthorized Access"

The problem was fixed!