Two-finger scroll needs three fingers
Summary
After rebooting the two finger scroll suddenly needed three fingers, and retriggering the dconf values in GNOME did not fix it. I use libinput since ages and today was the first time to hit it.
The suggested fix in https://bugs.launchpad.net/ubuntu/+source/libinput/+bug/1727541 helped for me as well. It seems nobody forwarded the issue there, that's why I do it now.
libinput version you encountered the bug on
libinput Debian package 1.12.6-1 Debian with Linux 4.19.0-3-amd64 (4.19.20-1) kernel
Hardware information:
Lenovo X250
Other log output:
In journalctl I find
libinput error: event17 - SynPS/2 Synaptics TouchPad: kernel bug: Touch jump detected and discarded.
See https://wayland.freedesktop.org/libinput/doc/1.12.6/touchpad-jumping-cursors.html for details