Mouse wheel button mapping is ignored

I am using ButtonMapping option in xorg xonfiguration.
I remapped my left wheel tilt to be my middle button, using this option: "ButtonMapping" "1 6 3 4 5 2 7 8 9". (basically button 6 and 2 are reversed).

And it worked fine - my left wheel tilt was acting like a middle button, which I use a lot for pasting - it's way easier to tilt the wheel, than to actually click it.

Unfortunately, after updating to qt-5.5.1 it doesn't work anymore. In qt5 apps as well as in Plasma 5 apps, left button tilt is treated as left horizontal scroll, no matter what.
Changing ButtonMapping affects everything else (qt4 apps, gtk apps), but not the apps that use qt5.

It is not just an issue with pasting. In the past, changing the horizontal scrolling orientation was possible by switching buttons 6 and 7 in button mapping. It no longer works in qt-5.5.1.

Reverting back to qt-5.5.0 restores the correct behaviour - both treating left tilt as middle button, as well as changing the scrolling orientation.

It also only seems to affect the wheel. For example switching buttons 1 and 3 works properly, but switching buttons 4 and 5 (vertical scrolling) doesn't.