For me, it happens with taxonmy fields, when term_id and term_taxonomy_id are different in the table wp_term_taxonomy. (the taxonomy is create by an import of data).
Eric
Hi,
In wp-content\plugins\advanced-custom-fields-pro\fields\taxonomy.php
try to change (near line 320):
$term_ids = wp_get_object_terms($post_id, $field[‘taxonomy’], array(‘fields’ => ‘tt_ids‘, ‘orderby’ => ‘none’));
by
$term_ids = wp_get_object_terms($post_id, $field[‘taxonomy’], array(‘fields’ => ‘ids‘, ‘orderby’ => ‘none’));
It works for me (version 5.1.4)
Eric