Dynamically loaded choices should not make a difference. Those dynamic choices should also be populated when you call get_field_object()
What type of field is it and what are the field settings?
To see what you’re working with
$field = get_field_object( 'custom_field' );
echo '<pre>'; var_dump($field); echo '</pre>';
$value = get_post_meta( $post_id, 'custom_field', true );
echo '<pre>'; var_dump($value); echo '</pre>';
echo $field['choices'][ $value ];