David,
It's not a bug as such.

The plugin works by creating two rectangles, the first one is the drag box marquee rectangle and the second is created from each object's 'extents'. If those rectangles overlap then the object is selected.
In your example, imagine the extents rectangle of the, 'polyline that should not be selected', and it can be seen that it would overlap your marquee rectangles.
The work around, if it's possible, would be to find a position for the marquee that did not overlap all the extents rectangles.
This is the piece of code;
if (dragBox.Overlaps(entityBox, 0.0))
The 0.0 is a tolerance value.