When a subfilter is active and some items are selected, a click on "Mass update" just resets the subfilter.
The items are still selected and the next click on the button performs the mass update.

Related:
When there are many items and pagination is taking place, then the final mass update seems only to update the items on the current page.
The pages have to be switched and the mass update has to be done for every page (items remain selected correctly).