Union of shapefiles produces wrong results

As mentioned here: https://github.com/MapWindow/MapWinGIS/issues/115

Unioning the two shapefiles results in 13 correct shapes en 2 wrong ones.

SoilTest:

UseTest:

Union result:

As you can see in the table editor the attributes of the last two shapes look weird.
And also the label of the second-last shape is out of the bounding box of rest and the last label is not shown.