The sort headers on tables like https://toolhub-demo.wmcloud.org/members are a great UX idea. Unfortunately right now they are also misleading to users. The sorts that they perform are local to the current page of results being displayed in the table rather than the global dataset behind the paginated set. Sorting this page by username as an example will order the rows on the current screen, but likely will not actually show you all the accounts starting with "A" when sorting the first page of results.
The fix for this issue would be to connect the sort order with the backend data query and issue a new query with the desired sort when the sort direction or column changes. This may or may not require adding some additional sort key support to the particular API endpoint backing a particular table, but generally that is a simple fix in the django code.