If the number of pages do not change, number of items displayed do not get updated until page size or page index is changed.
For example, if number of items is 15 and page size is 10, we have 2 pages. If after filtering we have 11 items, we still have 2 pages but less items. In this case, the text will still show "Displaying items 1-10 of 15" until we change the page or page size. If we go to the second page it will update correctly as "Displaying items 11-11 of 11" on the second page.