Custom search for a custom post type in WordPress

Change operator to IN:

array(
    'taxonomy' => 'features',
    'terms'    => $feature_slugs,
    'field' => 'slug',
    'operator' => 'IN',
),

That will query posts that have any of the terms in $feature_slugs, rather than all of the terms, which is what AND does.

Also, I can see you’re using the POST method for this search. That type of request is inappropriate here. When you are requesting data you should be using a GET request and accessing the data with $_GET[].

POST should only be used when sending data to the server to be processed, usually when it will create or update a resource.