Some boilerplate code for populating an ACF select box with custom taxonomies and then querying a custom post type using the result of that select box.

acf_populate_custom_tax.php

<?php
/* 

Assumes a custom post type called 'staff'
Assumes that there is a custom taxonomy called 'department'.
Every department that has at least one staff member assigned to it will
populate a select field that is specified in Advanced Custom Fields.

Using that Select field we can then do a query on a custom post type
in this case staff and grab only the staff with our selected department.

*/


// Part 1
// Place this filter in functions.php or another file of your choosing depending on project structure
// This filter uses the field key, for other options 
// check the ACF docs here -> https://www.advancedcustomfields.com/resources/acfload_field/
add_filter('acf/load_field/key=field_58b064ec5de3d', 'sby_department_acf_select');
function sby_department_acf_select( $field ) {

	$departments = get_terms( array(
	    'taxonomy' => 'department',
	    'orderby'    => 'name',
	) );

	foreach($departments as $department) {
		$field['choices'][$department->slug] = $department->name;
	}

    return $field;

}

?>

acf_query_custom_tax.php

<?php
// Part 2
// Place the following code where you want to do a query
// Assumes that this is in a flexible content custom field
$selected_department = get_sub_field('department');

$args = array(
     'post_type' => 'staff',
     'posts_per_page' => -1,
     'tax_query' => array(
             array(
                 'taxonomy' => 'department',
                 'field' => 'slug',
                 'terms' => $selected_department
             )
         )
);

$loop = new WP_Query( $args );

while ( $loop->have_posts() ) : $loop->the_post(); 
  the_title();
endwhile; 

/* Reset the post data because of our custom query */
wp_reset_postdata(); 
?>

Thank you, nice job!