I just noticed your filter
add_filter('acf/update_value/key=1_bild', 'acf_set_featured_image', 10, 3);
1_bild is not a valid field key, that is the field name, so your filter is probably never run. It should probably be
add_filter('acf/update_value/name=1_bild', 'acf_set_featured_image', 10, 3);
Also, not sure if this will effect anything or not but it’s generally not a good idea to use a field name that begins with a number. This all depends on what you plan to do with the name though so in most cases it won’t matter.
