A problem and a solution…
After installing the 5.13.16 kernel this morning (in Fedora-34, KDE spin), every application appeared in ALL activities. This really makes the very idea of activities useless, and has happened before after a kernel upgrade. in each case the problem disappeared after re-booting, so it’s easily mitigated.
It’s somehow related to a problem the Wayland version had initially, when it was impossible to use activities for this very reason. If it helps, the bug is probably in the window manager software invoked by right-clicking on the app icon to set the activity where the window is supposed to appear.