My setup: Laptop with 1680x1050 screen, when I connect my laptop to external screen (1920x1080) laptop screen turns off and I use solely external screen. If it matters - this is done by standard kwin_x11. I also have two activites - "work" and "personal", among the other things they have different wallpapers set up. Now, if I am using "work" activity and connect external screen - my other activity, "personal" will lose its wallpaper and use default one. Resetting wallpaper fixes problem until screens gets switched again (connect or disconnect doesn't seem to matter) - other activity would lose wallpaper again. After experiencing this problem one too many times I've inspected plasma-org.kde.plasma.desktop-appletsrc file and found out that it has, like 100 similar blocks of text, something like this: [Containments][21] activityId=a98eb87c-83e0-4d62-800f-3d7612158aaa formfactor=0 immutability=1 lastScreen=0 location=0 plugin=org.kde.desktopcontainment wallpaperplugin=org.kde.image [Containments][21][Wallpaper][org.kde.image][General] height=1080 width=1920 All of them were more or less idetical (only things deviating were resolutions - ether 1920 one or 1680 one and one of the two available activity ids).
After adding file desktop-appletsrc to a git repo I've discovered that behaviour on switching screens is something like that: 1) find containment for current activity wallpaper and change it's resolution 2) create a containment for other activity. (I'm not sure if this occurs on screen change or on activity switching after screen switch)
We did a bunch of multi monitor fixes last Wednesday that should end up in Plasma 5.8.4 . We fixed a bug with similar symptoms, so hopefully fixed this Can you reopen this if you still have problems after that release. Also if it does still happen you can you upload your ~/.config/plasma* files