I suggest a workaround might be to:
1. sudo apt install xserver-xorg-input-synaptics
2. Only log in to "Ubuntu on Xorg", so now you're avoiding libinput completely.
3. Tweak your settings with 'synclient'.
Since this sounds like a problem specifically with libinput, please help
us to progress this issue by reporting it upstream to the libinput
And when done, let us know the ID of the newly-created bug.
** Package changed: gnome-shell (Ubuntu) => libinput (Ubuntu)
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touchpad disable while typing, disable tap-to-click don't work in
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