When my Lenovo YOGA 710-15IKB is not converted into tablet mode, often touch input doesn't work. Rarely it does work, but I can't detect a pattern. However, when I convert the laptop by folding it all the way back into tablet mode, touch input works. On a default install of Fedora 29 gnome, touch input always worked. It didn't on Fedora 29 KDE spin, so it seems to be a KDE issue. (I'm now on Manjaro with kernel 5.0.5.)
KWin has nothing to do with touch input on X11.
Sorry, I may have even filed a bug about this before to Kwin. What is the right product and component to file this under?
Since you say "on X11", I should mention that the problem is the same on wayland.