This is by design. The Maintenance windows are still attached to the monitor and also "visible" in the UI, when you use their id in URL, such as
What makes it different to the one active,
is that that one still has a Time Window waiting (to be active in the future), while the former two don't have any Time windows to get active in the future. We call this Maintenance window as "passed" and in that case it is not show in UI, unless it's marked as "Persistent". If a Maintenance window has the persistent flag checked, it's displayed forever.