SUMMARY
This issue mainly concerns web browsers. For example, if you open all your music tabs in a window which is in the "intertainment" activity and all your coding related tabs in a window located on the "work" activity, thoose windows will all be opened in the same activity after rebooting and restarting the browser, loosing all your setup.

STEPS TO REPRODUCE
1. Open Firefox, Brave or Chromium, open a soundcloud.com tab for example, place it in an activity (entertainment for example)
2. Open a new window of your browser, open a github tab then ensure it's in your work activity
3. quit the browser (not window by window so it remembers that multiple windows were opened)
4. restart the browser

OBSERVED RESULT
All the windows are opened in the activity were you opened the app

EXPECTED RESULT
Windows that were placed in specific activities should be reopened there. If we are in an activity were no browser's window were opened a new one could be opened there or a notification displayed.

SOFTWARE/OS VERSIONS
Linux/KDE Plasma: Ubuntu 19.10 with backports repos enabled for plasma
(available in About System)
KDE Plasma Version: 5.18.3
KDE Frameworks Version: 6.67.0
Qt Version: 5.12.4

ADDITIONAL INFORMATION
Reboots due to software updates make this behavior really anoying