Well, there are many things happening with my dual monitor system. It happened one, two, three and etc. months ago with different versions of plasma (on Ubuntu, Kubuntu and Neon). In different combinations, but they did happen. Moreover - they're still happening. 1. At the moment I have to manually move my default panel and all widgets back from my secondary monitor to the primary one after EVERY reboot/logout. 2. Very rarely, but I get wrong resolution on my primary monitor. I have to adjust it manually via f.e. system settings or nvidia-settings. 3. Windows may open on those displays they want. Blender f.e. always opens on the secondary one. I can't change this behavior. It's hopeless. Steam games always do the same - wrong monitor. Applications seem to completely ignore my primary monitor settings made in KDE system settings and in nvidia-settings. At the moment I'm using nvidia 367 driver, but the same happened with 361, and 364... etc. My system: KDE Neon 5.7. Plasma 5.7.0. QT 5.7.0. KDE Frameworks 5.23.0.
I have the same issue in Plasma 5.8.0. My primary monitor is the right-most monitor. When I log in, the panel from my left-hand (non-primary) monitor is on the right-hand monitor and vice versa. The widgets from the left monitor are also on the right while the left monitor itself has no widgets at all and the default Plasma wallpaper. When I go to the screen system settings and change the primary monitor to be the left one and then back to the right one, the panels switch position, but the left monitor still has the wrong wallpaper and no widgets. I always have to correct all this after every reboot because when I log out and log back in, my monitor desktop setup is messed up again. This is super annoying. I thought multi-monitor issues were solved, but this is completely broken here. One interesting side node: I have two of these "Desktop" handles which open the plasma desktop context menu as if I had two plasma desktops on top of each other. One of them is clickable and movable, the other isn't (see screenshot).
Created attachment 102291 [details] Duplicate desktop handles
Additional information: when I switch the primary monitor, the panels do switch, that messes up the task manager. Windows from the left screen are shown in the task manager from the right screen vice versa. Manually moving the panels from on screen to the other works, but again, all that is gone when I reboot.
Looks like there are a couple of similar reports which look like the same problem to me. I'll mark them as duplicate. Reopen if you disagree. *** This bug has been marked as a duplicate of bug 371734 ***