See http://doc.qt.io/qt-5/qml-qtquick-controls2-rangeslider.html#first-prop and http://doc.qt.io/qt-5/qml-qtquick-controls2-rangeslider.html#second-prop

Both of these state:

If to is greater than from, the value of the first handle must be greater than the second, and vice versa. 

This is not true. It is the reverse. If to is greater than from, first.value <= second.value.