SUMMARY
Safe sessions does not honor the selection of activities when opening a saved session. Programs that should show on e.g. Activity one (at saving time) will just be opened all in the active activity when restarting the desktop. 
Hence the preferences for activities vs. open programs are lost while saving sessions. 
This makes activities in combination with save session a bit worthless. 

STEPS TO REPRODUCE
1.  open e.g. three activities. Open programs on several activities (which are visible only in that activities). 
2. save session
3. logout and restart plasma session. All settings that define preferences for the open programs are lost and all windows open indiscriminately in the first activity to open. 

OBSERVED RESULT
preference "per activity" vs open programs are lost when session is saved. 

EXPECTED RESULT
the session should be restored with the activities and the programs should open in the activities according to the preferences set and saved. 

SOFTWARE/OS VERSIONS
Operating System: openSUSE Tumbleweed 20220207
KDE Plasma Version: 5.24.0
KDE Frameworks Version: 5.90.0
Qt Version: 5.15.2
Kernel Version: 5.16.5-1-default (64-bit)
Graphics Platform: X11
Processors: 4 × AMD FX(tm)-4300 Quad-Core Processor
Memory: 31.3 GiB of RAM
Graphics Processor: AMD Radeon Pro WX 3200 Series