Wacom tablet's stylus buttons can not be configured

[Marcelo]

The functions assigned to a wacom tablet's stylus buttons using SystemSettings -> Input Devices -> Tablet (translated from portuguese, names in english might not be exactly those) are ignored by the system and buttons 2 and 3 always produce a left-click. Everything else about the tablet works fine (pressure sensitivity, assigning functions to the tablet's express buttons, etc.).
If xserver-xorg-input-wacom-lts-wily is removed, the wacom tablet is not detected as a tablet anymore (as expected) and does not show up in SystemSettings -> Input Devices -> Tablet, but it can still be used as a mouse and, in this case, the stylus buttons work as middle-click and right-click.
I'm running Kubuntu 14.04.4 LTS (fresh install) with the kubuntu backports PPA enabled. KDE Version is 4.14.13.
The wacom tablet is a Intuos Pro Small (PTH-451).
xserver-xorg-input-wacom-lts-wily version is 1:0.30.0-0ubuntu3~trusty1.

ProblemType: Bug
DistroRelease: Ubuntu 14.04
Package: xserver-xorg-input-wacom-lts-wily 1:0.30.0-0ubuntu3~trusty1
ProcVersionSignature: Ubuntu 4.2.0-35.40~14.04.1-generic 4.2.8-ckt5
Uname: Linux 4.2.0-35-generic x86_64
NonfreeKernelModules: fglrx
ApportVersion: 2.14.1-0ubuntu3.19
Architecture: amd64
CurrentDesktop: KDE
Date: Sun May 1 21:11:20 2016
InstallationDate: Installed on 2016-05-01 (0 days ago)
InstallationMedia: Kubuntu 14.04.4 LTS "Trusty Tahr" - Release amd64 (20160217.1)
SourcePackage: xf86-input-wacom-lts-wily
UpgradeStatus: No upgrade log present (probably fresh install)