Myeah...pain the a.. . Sometimes works, sometimes doesn't. Anywho...if you set save session manually in system settings, you get this option in the logout menu.
https://imgur.com/a/De05UHe

Me personally, I don't save sessions. But I remember this issue and I made a desktop application for that once. It overrides the already saved session by creating a new one. Seemed more logical to me. Remember though. Certain apps aren't kde backed and, as such, won't save. For example, Chrome, Audacious... You'll have to find out for yourself which ones you use and don't get saved. Those you'll have to add to your autostart if you want them to launch on startup.

https://imgur.com/a/SDGKlGS

So, the command: qdbus org.kde.ksmserver /KSMServer org.kde.KSMServerInterface.saveCurrentSessionAs "saved at previous logout"