A table with class="wikitable-sortable" is not displayed as expected when the last row consists only of cells filled via rowspan from previous rows and header cells.

I have provided a variety of tables at my sandbox to demonstrate the problem (tested in Vector skin; did not test other skins).

The first table is broken; the "A" should extend over both rows and the "2" should be under the "1".
Changing to class="wikitable" works as expected.
Changing the non-rowspan cell in the last row to a non-header works as expected.
Changing the rowspan in the first column to two separate entries works as expected.
Adding another column with a non-header, non-rowspan cell in the last row works as expected.