When the xf86-input-libinput driver is installed, it will often end up stealing Wacom devices like touchscreens and pads away from the xf86-input-wacom driver. This prevents the devices from showing up in xsetwacom, can prevevent their configuration through the GNOME control center, and prevents the X driver's pen/touch arbitration from working (though kernel arbitration is usually still available).
This is caused by xf86-input-libinput having a "90-libinput.conf" in xorg.conf.d which the X server uses in preference to our own "50-wacom.conf".
Fixed xf86-input-wacom by commit 0da5cd5 (expected release: xf86-input-wacom 0.33.0)
Also requires update of xf86-input-libinput (commit 3f569ec; xf86-input-libinput 0.19.0)
Fix available through a combination of xf86-input-wacom 0.33.0 and xf86-input-libinput 0.19.0