Thanks for your efforts. I updated to rc15.
Not only a UI issue. Set Tags to ‘Not one of…’ and click ‘Export’. The code ‘tag_operator’ => ‘tag__not_in’. Looks good.
However, when you look at the resulting query object, it turns out that all the ticked tags are still being outputted in the ‘tag__in’ array and nothing in the ‘tag__not_in’ array.