I've been searching for more information about this bug, and I've found a bug report at RedHat that probably is connected to this one: https://bugzilla.redhat.com/show_bug.cgi?id=1227522 . This had led me to another bug report, this time, in KDE: https://bugs.kde.org/show_bug.cgi?id=350688 .
So it seems to be an upstream bug between KDE Plasma and LibInput. As a workaround, one can use the "xinput" command to set the acceleration directly from the command line: xinput set-prop <device_id> 276 <value> 276 is the ID of the property "libinput Accel Speed". For example, in my case, to set the acceleration to the maximum (1.0), I've had to do this: xinput set-prop 12 276 1.0 Use this command to list the connected devices: xinput list Use this command to know the current properties of a specific device: xinput list-props <device_id> Another workaround is to just install "xserver-xorg-input-evdev" and uninstall "xserver-xorg-input-libinput". In my opinion, for now, EvDev suits my needs much better than LibInput. Salutes.
