Hi,
You are getting the value of your custom field (jobs_kategorie) outside of loop so you should provide the post id as well in get_field() like get_field('category-name', 64)
.
For your case I recommend you to add another custom field for post id.
$wp_query = new WP_Query (array('post_type' => 'stellenangebote', 'category_name' => get_field('jobs_kategorie', 'jobs_post_id') ));
?>
Regards,