My non-corporate desktop has 3 screens, screen#2 is shared with a corporate laptop (via a KVM, but the issues happen without it as well).

If I switch that monitor to the other machine, Windows re-arranges ALL windows to appear on the new "primary" portrait-oriented screen#1, some maximised to fill the screen, some not. They stay there after the other screen is reconnected.

Possibly because the screen being switched is the "primary" screen? At least it's consistent behaviour between both Win10 AND Win11, which is nice.