I am using Outlook v16.16.17 on macOS 10.14.6. The Outlook search (top right corner of Outlook, not Spotlight search) used to return everything. At some point, it started only returning items from the currently active folder (the folder selected in the sidebar). For example, if I am in Inbox, it will not return items from Sent. To search sent items, I have to go to Sent and then repeat the search. At first I thought it was some random glitch, but after several months and multiple updates and restarts, it continues to happen.
I saw some similar posts (such as How to fix Outlook for Mac search), but they seem to be related to Spotlight indexing or privacy. I don't think they are related. The Spotlight search actually works, so messages are definitely indexed.
I also have access to a web-based Outlook interface to the same account. The search works across all folders there, so it's not some kind of account restriction.