With .htaccess inheritance, it forces the current configuration to inherit or obtain the parent directory’s configuration. In per-virtual-server context, it means that maps, conditions, and rules of the main server are inherited. In per-directory context, it means that the conditions and rules of the parent directory’s .htaccess configuration are also inherited.
Rules inherited from the parent scope are applied after rules are specified in the child scope. Making the child directory (addon or subdomain) would take the .htaccess of their parent directory (main domain).
To resolve this, you can create a new .htaccess file in the child directory and place the following code in: