I have 3 activities in my plasma5-desktop-5.18.6-bp153.3.2.1.x86_64.
I have a activity manager in the bar.
Recently It has happend twice the same.
I boot and my desktop is bad configured: the wallpapers are gone, the icons have lost their position.
I notice that the activity manager is not there.
I log out and log in again … or reboot. The activity manager is again there, but the configuration is lost.
Somebody is experiencing something similar?