This may be a feature rather than a bug, but it doesn't make sense to me.  If I turn on window grouping (system settings -> window behavior -> window behavior -> advanced -> automatically group similar windows) grouping works fine.   However, I then have the following problem.

In activity1, I'll open Konsole and start a process.  Then I'll move to activity2 and find I need a terminal, so I'll open Konsole -- but instead of opening a Konsole in activity2, it opens one in activity1 and groups it with the first Konsole.  This may be the way it's designed, but it would be more convenient if grouping occurred within a single activity rather than crossing activities.  This way, I can have an active terminal in only one activity if grouping is on.

Interestingly, this seems to happen only with Konsole so far -- it doesn't happen with Firefox or any of the other apps I have tried.

My workaround is either to turn off grouping or use a different terminal app for each activity, e.g. konsole for activity1 and gnome terminal for activity2.