Not true. It’s just not as consistent as using Intel or AMD graphics.
Did you try to use x11(Xorg) when using KDE?
Wayland, though it is default on many distros, is actually experimental. At the very least, it’s seeing heavy amounts of development that can cause random issues on random devices/hardware.
You may want to have a look at: Plasma 6 Update some hints
I have no experience using an external monitor on Linux, so someone else will chime in on that.