This is Dash Core version v0.12.1.5 (64-bit) download?
Dogecoin has a similar issue, though in their case the wrong sort order is lexicographic and the comments suggest that locale setting may be the cause.
https://github.com/dogecoin/dogecoin/issues/1289
Sorting is handled by QT. In dash-qt, an extra column is added just for the purpose of sorting
(containing the column text padded to the left to a fixed width so that lexicographic order is numerical order).
The QT documentation is vague about sorting. This QT forum link gives some more useful information.
https://forum.qt.io/topic/61239/how-does-qtreewidgetitem-sortchildren-sort/2
(Bitcoin-qt wallet uses setData and UserRole on the items, as suggested in that link, rather than using an extra column.)
(Sorting by the 'model' data itself may be a more correct approach, though our data is grouped by address.)
This is probably a 'red-herring' - in your screenshot the (trailing) rows with PS rounds are sorted while the (leading) rows with PS rounds n/a are sorted if the group with the same 'Received with address' are represented by the first group item.