With the latest DX SaaS release, if an agent is down for a specific period of time, you can go back and view the details in Metric view by picking up a filter for a specific day and time. This feature is not working in below scenarios:
Release : DX SAAS 20.4
Per APM Product Management and Dev:
A disconnected Agent won't show if the Agent was connected and disconnected completely within the historical time window, meaning time-window-begin-time < agent-connect-time < agent-disconnect-time < time-window-end-time
By moving the timeline to a past time interval and viewing an agent connected at that time, you can now view the corresponding metrics in the map contextual view.
These are new, changed, and deprecated features in DX APM 20.4