SUMMARY
I have 3 activities where are placed windows. I work with X11 Graphics Platform, because in Wayland still doesn't work restore session, and not sure about activities.

First start of Plasma 5.25 shown me all windows in "Default" activities. After moving first window into other activity normal managing windows becomes very hard. Recently clicked window behaves like could be "always on top". Switching between windows clicking into any becomes impossible. Switching between windows by Alt+Tab works. After minimize all widows and assign all to proper activities and relogin shows again al windows in one activity.
After moving one windows to any activity showing launcher menu or activities bar is not possible. They are shows underneath of opened windows. The work around is minimize all windows.

I removed all activities,  relogged, added again activities and trying put windows onto proper activity met above described behaviour.

IMO this looks like disaster for someone who use activities daily! 

As far as I remember, such behaviour happened in Wayland session, when I tested it couple  months ago.

STEPS TO REPRODUCE
1. Open several applications
2. Add one or two activities
3. Move one window to one of added activity
Being on "Default" activity
4. Try to click onto any application placed on task bar to raise it
5. Try to click onto opened window (in Desktop) to raise it
6. Try to open launcher menu or activity side bar
7. Minimize all and add  couple applications  to proper activities and relogin (restore session must be turned on)

OBSERVED RESULT
Completely broken managing of windows, seems like one or more behaves like could be "always on top" , 
Switching windows by click onto window placed on desktop or placed in task manager doesn't work. I mean it makes that window is raised underneath currently activated, so is not possible to get clicked one.
Restore session doesn't work - all windows are placed in one (Default) activity

EXPECTED RESULT
Possibility of switching of windows like in Plasma 5.24, 
Restoring session restores all windows to all be placed in assigned activities

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: 
(available in About System)
KDE Plasma Version: 5.25
KDE Frameworks Version: 5.95
Qt Version: 5.15.5 with recent KDE patches

ADDITIONAL INFORMATION