SUMMARY
The issue here is that if, for example, I have 10 Firefox windows open on one desktop, and I then asign one Firefox window to Desktop 2, and another Firefox window to Desktop 3, then logout and login again, the separation of the Firefox windows is lost:  the fireox windows are not ***all*** located on Desktop 3.  I have tried this time, and time again, with the same result each time.

Using Plasma 5.15.4 on Mageia 7.  Up to date on all packages.

STEPS TO REPRODUCE
1. Boot Mageia 7 and open a Plasma KDE session.
2. Create new Desktops (Desktop 2 and Desktop 3)
3. Settings: Desktop Behaviour -> Activities -> Switching:  Select "Remeber the current virtual desktop for each activity"
4. Open Firefox
5. Visit a website on that page.
6. Open a new Firefox window and visit a website on that page, then Right-click that page (in the menu) and select 'Move to Desktop' and select 'Desktop 2'.
7. Open another new Firefox window and visit a website on that page, then Right-click that page (in the menu) and select 'Move to Desktop' and select 'Desktop 3'.
8. Logout
9. Login again

OBSERVED RESULT

  All 3 Firefox windows will be found on Desktop 3. ie the Desktop to which the most recently transferred windown of that app is assigned, is the Desktop on which all windows of that application appear upon login again.

  Note: other applications that I have open at the time on Desktop 1, remain on Desktop 1 after logging out and logging back in again, despite the movement of all windows of Firefox to the Desktop to which a Firefox window was most recently assigned.  

EXPECTED RESULT

  Window 1 of Firefox should remain on Desktop 1, Window 2 of Firefox should remain on Desktop 2, Window 3 should remain on Desktop 3.

SOFTWARE/OS VERSIONS
Windows: 
macOS: 
Linux/KDE Plasma: Mageia 7 / KDE Plasma 5.15.4
(available in About System)
KDE Plasma Version: 
KDE Frameworks Version: 5.57.0
Qt Version: 5.12.6

ADDITIONAL INFORMATION