Setting the Relational Post Object to write back to the post (the german translation for this function is not entirely cleat BTW), changing the taxonomy selector to Radio Button (drop down seem to have a bug, as it mixes taxonomies translations) and opening and re-saving every member CPT did the trick, with the following code:
$tax = get_field_object("department");
$taxnum = $tax['value'];
$taxtax = $tax['name'];
$dept = get_term( $taxnum, $taxtax );
$deptname = $dept->name;
Now for the same fun in a repeating field group… yay!
