WordPress-site can be reached on x.x.x.x/index.php – but not directly on the IP without index.php

Is this normal behaviour?

Yes. On a default Apache install, only index.html is set as the DirectoryIndex (the default setting).

Either in the appropriate <VirtualHost> container, or main server config you need to add a <Directory> section that specifically targets the defined document root directory.

For example:

DocumentRoot /var/www/html

<Directory "/var/www/html">
    # Allow public access to the site
    Require all granted

    # FollowSymLinks is required for mod_rewrite
    # (Although this is actually the default server setting)
    Options FollowSymLinks

    # Allow mod_dir to serve index.php by default
    DirectoryIndex index.php

    # Give full access to htaccess (optional)
    # Strictly speaking you only need "FileInfo" for mod_rewrite
    # - Alternatively place all your htaccess directives here...
    AllowOverride All
</Directory>

You will then need to restart Apache.

mod_dir also needs to be enabled, but this should be loaded by default.