The ibexa/post-install package includes the default server configuration for Apache and Nginx where the following regexp is used to prevent accessing index.php directly:

^/([^/]+/)?index\.php([/?#]|$) 

This regexp will match the following:

/index.php
/wp-content/index.php

But it will not match if there are more than one "sub-part", ie:

/wp-content/themes/twentystd/index.php

The URL will resolve to the root of the site access (depending on how it's configured) with broken URLs for all contents (CSS & JS files, images, etc...)

To resolve the issue we just need to specify the the first capturing group of the regexp should be repeated with * :

^/([^/]+/)*?index\.php([/?#]|$) 

I think this issue might be related to EZP-25057, EZP-27285 and maybe other I didn't find.  But since they are very old I decided to create a new one with a possible fix.

I will create a merge request on github with the fix and post it here in the comment