Can't see calendar when viewing invites in Inbox

I support several people in my department and up until about a week ago I could see their calendar in the same window whenever I clicked on a meeting invite in my Inbox. Now I have to manually click the option to view their calendar to look for conflicts, which is really inconvenient. 

I don't know if I hit a key combination while in my Inbox because it was working just fine and then all of a sudden it wasn't. We hadn't had any Office 365 updates from when it was working to when it stopped working. It seems like there must be an on/off feature for this, but I'm not finding it.