I am currently running Plasma 6.5.3 using the Fedora KDE 42 distro. However, I had Kubuntu installed previously, and I had a similar problem. I have a 3-monitor setup with all monitors in a horizontal row.
The problem I have is that the 2nd monitor consistently offsets all windows from the top of the screen about the width of the Titlebar. There is usually (but not always) a gap between the top of the window and the top of the screen. The wallpaper is visible in the gap. I could live with that, but the mouse does not seem to be offset. If I need to click anything in the title bar or the upper part of the screen, I have to mentally adjust for the offset. So, for example, I have the X to close the window in the top right of my browser. Right below the X there is the browser settings dots. If I click on the X to close the window, I get the dots. If I click on the dots, I get the bookmarks which is right below the dots. To close the window I would have to click on the wallpaper. It gets very touchy at times and I often end up clicking different buttons, or I have to click around in a small area to try to hit the offset sweet spot.
Strangely, it does not happen on every boot; sometimes it works correctly. Sometimes I get the same effect on the 3rd monitor. I don’t think I have ever had the problem on the first monitor.
I have tried a variety of settings tweaks. I may not be searching the web with the correct terms, but I am not finding anyone else with this problem. That is odd because I have had an identical problem on two different distributions (Kubuntu and Fedora).