You cannot. It’s a bug in Plasma itself, but it manifests only on X11.
On Wayland, the clipboard behavior in Plasma 6.0.5 is even worse. If you highlight something with the mouse, then your cllipboard will be filled with every step of the highlighting.
In other words, say that you highlight the word “Manjaro” with the mouse by marking it — not double-clicking. The clipboard will then contain the following entries…
Manjaro
Manjar
Manja
Manj
Man
Ma
M