QT_SCALE_FACTOR + libinput = wrong touch coordinates

QT_SCALE_FACTOR works very well with evdev and tslib. But using this macro with libinput results in wrong touch coordinates.

Example:
If I have a physical screen size of 800x600 and set QT_SCALE_FACTOR = 2, then the resulting viewport size is 400 x 300. So far so good.

When I touch in the middle of the screen, the touchpoint is received at (100, 75) instead of (200,150).

If there is no scaling active, libinput delivers correct values.