After upgrading to Tumbleweed 20220616 with KDE Plasma 5.25, if you find all your virtual desktop windows restored onto your first virtual desktop, it’s probably this bug: https://bugs.kde.org/show_bug.cgi?id=442380
A work around is to disable Plasma’s use of systemd boot. When logged into the problem session, run the following command:
kwriteconfig5 --file startkderc --group General --key systemdBoot false
After that, future logins should revert to the correct behaviour.