Why is AcceptPathInfo applied when changing the filename, yet isn’t when I change the path? More concretely, I can get it to rewrite with:

RewriteCond %{REQUEST_FILENAME} -f
RewriteCond %{REQUEST_FILENAME}.php -f
RewriteCond %{REQUEST_FILENAME}:%{DOCUMENT_ROOT}%{REQUEST_URI} (.*):g1(.*)
RewriteRule ^ %{REQUEST_FILENAME}.php%2 [END]

And given that /aaa.php exists, I get:

https://example.com/aaa/bbb/ccc -> https://example.com/aaa.php/bbb/ccc

Yet when I try it with:

RewriteCond %{REQUEST_FILENAME} -f
RewriteCond %{REQUEST_FILENAME}/index.php -f
RewriteCond %{REQUEST_FILENAME}:%{DOCUMENT_ROOT}%{REQUEST_URI} (.*):g1(.*)
RewriteRule ^ %{REQUEST_FILENAME}/index.php%2 [END]

And given that /aaa/index.php exists, I get:

https://example.com/aaa/bbb/ccc -> 404 Not Found

I tried logging with LogLevel alert rewrite:trace6, but all I saw is that RewriteCond %{REQUEST_FILENAME}.php -f was checked for %{DOCUMENT_ROOT}/aaa.php, as if it automatically tried all potential pathnames in case of %{REQUEST_FILENAME}.php, but not %{REQUEST_FILENAME}/index.php.

Is it possible to get this to work? That given /aaa/index.php existing I would get:

https://example.com/aaa/bbb/ccc -> https://example.com/aaa/index.php/bbb/ccc

Just like the .php rewrite above.