Subfolder index.php 404 Errors

I’m looking for help troubleshooting why subfolder pages return a 404 error if “index.php” is appended after the subfolder. I do not experience this behavior on other WordPress sites. I have inherited this site from previous developers, so I don’t know what may have caused this.

Here are some examples and additional information:

WORKS:
https://www.mydomain.com/contact/
FAILS (404 PAGE):
https://www.mydomain.com/contact/index.php

PERMALINK SETTING:
Custom Structure: https://www.mydomain.com/%category%/%postname%/

HTACCESS:
<IfModule mod_rewrite.c>
RewriteEngine On
RewriteBase /
RewriteRule ^index\.php$ – [L]
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule . /index.php [L]
</IfModule>

Thanks!