I have encountered a bug where the touch location is in the wrong position when using an extended desktop. Normally the touch works fine with the laptop display. When extended displays are used, it the touch calibration includes both monitors. The result is if you touch the center of your laptop monitor, it selects near the side of the other display.
For example, if normally your displays are 1920 wide, it treats the touch calibration on the computer monitor as 3840 wide display.
This might be a KDE issue, but is it possible to make touch screen only respond to a single display?