Descending sort does not work properly

When sorting on a column table column contains null values and using pagination, sorting in descending order does not work properly. When I choose a page size that shows all the values and sorting in descending order, the null values are shown at the top of the list. However, when I smaller page size (i.e. 25 or 50 per page), the data is sorting on the first page and that null values are spread between the last two pages (depending on number of data values). When sorting in ascending order, all the pages are sorted correctly (all null values are at the bottom).