Safari tabs rarely restoring to their respective desktops after restarting
Apps cannot be assigned to multiple Spaces.

If you split multiple windows of any single app to to separate desktops with no other pre-assignment, they will all open on the desktop that just happens to be frontmost when they launch. This can feel more random at boot time, when the screen is rapidly changing Spaces to place each already-assigned app on its correct Space.

You can fix any app to any Space by right clicking its icon in the Dock, then Options > 




All Desktops - the app will follow you wherever you go  
This Desktop - will change to show the Space number once assigned, then will always open in that Space.  
None - will open in whatever Space you're in at the time.


If you have Mission Control set to 'Displays have separate Spaces' &amp; 2 monitors, you will need a minimum of 3 Spaces before these options will become available to you.