Not really a KDE thing but having multiple screens with differing refresh rates is a pain on all distros I've tested. Either you add a lot of buffering to avoid screen tearing, or you get screen tearing.
I use KDE Plasma on Fedora, Wayland, and it works in the latest kernels. I remember it was an issue for me before, but now I have my laptop screen (120Hz) and desk screen (60Hz) working well.