In the new Outlook: When the Calendar app is opened up, the agenda pane does not populate info from "People's Calendars" unless those calendars are unchecked and then re-checked. That info needs to be populated automatically when the Calendar app opens -- otherwise, users who rely on the agenda pane for viewing the day's events will miss events shared by other people's calendars.
Kenneth Richardson commented
I completely and wholeheartedly agree. It became an irritant immediately as I often mistook that column as Saturday instead of just an open pane.
Recent updates to the application changed how the agenda pane works. In the Outlook Web App Calendar, the agenda pane opens whenever I start Outlook in my browser. I want to keep them closed by default like how it used to work but there is no setting for this.