This problem already happened twice on Neon unsrable edition.
First time it happened after I install Audacity audio editor, yesterday
it happened again after I install Signal Desktop (messenger).
STEPS TO REPRODUCE
1. start Plasma session (I used X11 session yesterday) on a laptop running a system where none Snap package was ever installed
2. make sure "Tap-to-click" feature is enalbed in system settings > input devices > touchpad
3. open Discover and search for "snap"
4. install a snap package
5. when installation is completed, try to click on something using "tap to click"
feature of your touchpad
nothing happens. "Tap to click" feature works again after relogin.
"Tap to click" feature of touchpad should work after the first snap package to be installed on the system.
Operating System: KDE neon Unstable Edition
KDE Plasma Version: 5.18.80
KDE Frameworks Version: 5.69.0
Qt Version: 5.14.1
I've seen other reports of touchpad settings resetting after various package management operations. I have *no idea* how these could be related. But it seems like something is going on.
Aaaaand now I just experienced it myself while doing an update of my Tumbleweed system using Discover. Settings were reset to their default values, but returned to my custom settings after a reboot. I was not using any Snap packages.
This is really weird.
Also experiencing on Solus 4.1 Plasma
Let me know if any logs might be of use, happy to provide.
*** This bug has been marked as a duplicate of bug 414559 ***