Comment 28 for bug 1715479

you're still using the synaptics driver? if you do an xinput list-props "device name", are the devices prefixed with Synaptics? or still Evdev like in comment #4? If so, you need to switch to the libinput driver. see comment #5. If you have ever had a mate or cinnamon package installed, that could be the reason, they unconditionally overwrite (or used to overwrite) the normal configuration. Look for any symlinks in /etc/X11/xorg.conf.d/