KDE is an international technology team creating user-friendly free and open source software for desktop and portable computing. KDE’s software runs on GNU/Linux, BSD and other operating systems, including Windows.
If you encounter a bug, proceed to https://bugs.kde.org, check whether it has been reported.
If it hasn't, report it yourself.
PLEASE THINK CAREFULLY BEFORE POSTING HERE.
Developers do not look for reports on social media, so they will not see it and all it does is clutter up the feed.
If you're on Wayland, there should be a toggle for it in the KDE settings.
For me on Plasma 6, it's called "Middle Click:" and then "Pastes Selected Text", under "Workspace" → "General Behaviour".
If you're on X11, this is implemented in X.org, so X.org would need to provide that option, not KDE. I researched that for half a minute and it didn't look great, but yeah, you'll want to do your own research.
Or try out the Wayland session, if you haven't yet.
Try "keyboard shortcuts" and select some action, then use the button as input, or look in the mouse settings.
Is there a "nothing" action? May be useful here.