Try using the update_field
ACF API function. In my experience, the backend ACF UI wasn’t selecting the attached terms because they weren’t associated correctly in the database.
update_field documentation:
https://www.advancedcustomfields.com/resources/update_field/#usage
Here is an example that uses a Gravity Forms hook. After a user submits the form, a new post is created. Once it’s created and the terms are attached to the post, I take those terms (array) and use the update_field ACF function to attach them. Now, my taxonomy checkboxes on the backend are correctly selected.
add_action( 'gform_after_submission_1', 'update_acf_terms_for_rentals', 10, 2 );
function update_acf_terms_for_rentals( $entry, $form ) {
//get the post
$post = get_post( $entry['post_id'] );
// get the term IDs array (IDs only)
$term_list = wp_get_post_terms($post->ID, 'space', array("fields" => "ids"));
// Insert the $term_list into the database via ACF
$field_key = 'field_5b50eb94713f1';
update_field($field_key, $term_list, $post->ID);
}