For some reason, the order of rows gets reversed.
To reproduce, simply …
Enable stable sorting in this example: dhtmlx.com/docs/products/dhtmlx … group.html
Group by bestseller
Sort by shipping
Sort by author
… and you will see that the order of rows for “Stephen King” is the opposite of what one would expect. (Especially noticeable also by sorting descending by shipping first and then author)
Ungroup and it works as it should.