Milestones allow alphabetical sorting by name. When the sort button is hovered over with the mouse pointer, a tooltp with a message about the sort direction is shown.

However, when sort order is changed by clicking on the sort button, the tootip continues to be present and every subsequent click on the sort button only changes the message inside the tooltip based in the current sort order.
The only way for the tooltip to disappear is to click anywhere outside sort button.

It is expected for the tooltip to disappear once the mouse pointer hover away from the sort button.