Let a facet show all options instead of the ones based on the currently filtered results.

<?php
// Let a facet show all options instead of the ones based on the currently filtered results.
// This changes the SQL that loads the facet values.
// Only for use in highly customized situations, doing this can have unexpected side effects.

add_filter( 'facetwp_facet_where', function( $where_clause, $facet ) {
    if ( 'my-facet' == $facet['name'] ) { // change 'my-facet' to the name of your facet
        $post_ids = FWP()->unfiltered_post_ids;
        $post_ids = empty( $post_ids ) ? [ 0 ] : $post_ids;
        $where_clause = ' AND post_id IN (' . implode( ',', $post_ids ) . ')';
    }
    return $where_clause;
}, 10, 2 )