The other day, I logged in to the XFCE4 environment instead of the usual KDE/Plasma, and made a few setup changes there (mainly just desktops, setting different images for the different desktops and monitors). Then I logged back into KDE/Plasma, and encountered two problems:
All of the windows that were started were in all activities.
The right click option to select which activity to show the windows was absent.
I assume that this must be some nasty configuration clash between XFCE4 and KDE/Plasma.
Since I don’t habitually use XFCE4, I removed the environment from my install, and then after also logging out and logging back in the menu item to assign windows to activities was restored.
I’m sure that this is an overkill way to solve the problem, but I thought I’d post it just in case anyone else hits the issue. I’d suggest trying logging out of the crippled plasma and logging back in without removing XFCE4 as a first resort.
If anyone does have better suggestions or explanations, I’d like to hear but this is mainly as a marker.