A taxonomy field returns an array of terms if your field settings allow selecting more than one term. In this case you have to loop over that array
$terms = get_field('cast_category');
if( $terms ) {
foreach ($terms as $term) {
?><h2><?php echo esc_html( $term->name ); ?></h2><?php
}
}
https://www.advancedcustomfields.com/resources/taxonomy/