See my comments below
// you don't need this, it's already done for this template
//global $post;
$bookingstart = get_field('booking_date_start');
$bookingend = get_field('booking_date_end');
$travelstart = get_field('travel_date_start');
$travelend = get_field('travel_date_end');
$promocode = get_field('booking_promo_code');
$relatedposts = get_field('applicable_resort',$post->ID);
if( $relatedposts ) {
// change this line
// then use $post instead of $relatedpost
foreach( $relatedposts as $post):
setup_postdata($post);
$resortname = get_the_title($post->ID);
$location = get_post_meta($post->ID,'location',true);
$logo = get_field('logo');
$demoterms = get_the_terms($post->ID, 'demographic_category');
if ( !is_wp_error($demoterms)) {
$demographic_category_nolinks = wp_list_pluck($demoterms, 'name');
$demographic_category_list = implode(", ", $demographic_category_nolinks);
}
endforeach;
}
// important, reset search loop
wp_reset_postdata();
