Cacher is the code snippet organizer for pro developers

<?php
/** index ACF Multi Dates Field picker addon fields which are saved as an array **/

add_filter( 'facetwp_index_row', function( $params, $class ) {
    if ( 'MY_FACET' == $params['facet_name'] ) { // change MY_FACET to name of your facet
        $values = maybe_unserialize( $params['facet_value'] );
        if ( is_array( $values ) && ! empty( $values ) ) {
          foreach ( $values as $value ) {
                  $new_params = $params;
                  $new_params['facet_value'] = $value;
                  $new_params['facet_display_value'] = $value;
                  $class->insert( $new_params ); // insert new value to the database
          }
          $params['facet_value'] = ''; // skip indexing original
        }
    }
    return $params;
}, 10, 2 );