Batch Extract Date from post title and put into ACF custom field

OK, so here’s the function that should do the trick:

function modify_events_to_use_new_field() {
    $events = get_posts( array(
        'post_type' => 'event',
        'post_status' => 'any',
        'posts_per_page' => -1
    ) );

    foreach ( $events as $event ) {
        if ( preg_match('@^(.*) - ([^-]*)$@', $event->post_title, $matches) ) { 
            // you'll have to modify the POST_META_KEY to the real name of your custom field
            update_post_meta( $event->ID, '<POST_META_KEY>', date('Ymd', strtotime($matches[2]) ) );
        }
    }
}

Now you have to change the name of custom field in there and run that function.