Ok found it… I was messing around with the query for categories (making it return my custom post type: course).
Had this in my functions:
function alter_category_query($query) {
if(is_category() && empty($query->query_vars['suppress_filters'])) {
$query->set('post_type', array('course'));
return $query;
}
}
add_filter('pre_get_posts', 'alter_category_query');
changed it to also include the acf-field post type:
$query->set('post_type', array('course', 'acf-field'));