Most of the time when I restart my computer, plasma looses the configuration of the wallpaper and tweaks I have set on my secondary monitor. When I log in I have to disable the desktop toolbox and set my wallpaper if I want it to have the correct settings. It seems to keep the settings after I upgrade the system, but it does not keep the settings when I do a normal shutdown.
I have this too after having a fresh install of Kubuntu 18.04. I have a laptop where I often use an external monitor as my primary monitor. I have added the cpu and memory widget to my secondary screen (i.e. laptop screen). I also set a different wallpaper. When I re-login I might get back the original screen or I might not. It seems to be random. It seems like the ID that the secondary monitor gets assigned in the plasma-org.kde.plasma.desktop-appletsrc file is not stable. I'm not entirely sure how to read that file though, so I could be wrong. But if I add a clock widget after my secondary monitor failed to display my previous widgets and look at that file, I see my original widgets and wallpaper as [Containments][2][Applets][22] immutability=1 plugin=org.kde.plasma.systemmonitor.memory [Containments][2][Applets][25] immutability=1 plugin=org.kde.plasma.systemmonitor.cpu [Containments][2][Wallpaper][org.kde.image][General] Image=file:///usr/share/wallpapers/Dance_of_the_Spirits/contents/images/1920x1080.jpg which all seem to be assigned [2]? But the newly added clock widget is [Containments][27][Applets][28] immutability=1 plugin=org.kde.plasma.analogclock So it's attached to [27] somehow.
Created attachment 112565 [details] plasma-org.kde.plasma.desktop-appletsrc My plasma-org.kde.plasma.desktop-appletsrc after adding a clock widget to the secondary monitor.
*** This bug has been marked as a duplicate of bug 393881 ***
*** This bug has been marked as a duplicate of bug 413645 ***