Untitled

<?php
/** custom set a facet to a specfic value on FWP.reset() instead of clearing
 ** change 'categories' to the name of your facet and 'post-formats' to
 ** the value you want selected
 **/
add_action( 'wp_footer', function() { ?>
    <script>
    (function($) {
        if ('object' !== typeof FWP) {
                return;
        }
        $(function() {
            FWP.hooks.addAction('facetwp/reset', function() {
                FWP.facets['categories'] = ['post-formats'];
            });
        });
    })(jQuery)
    </script>
<?php }, 100 );