<?php
// Add the following to your (child) theme's functions.php
add_filter( 'facetwp_index_row', function( $params, $class ) {
if ( 'region_partner' == $params['facet_name'] ) {
$params['facet_name'] = 'region_dropdown'; // trick FacetWP into saving values into the other facet
}
return $params;
}, 10, 2 );