This is a known issue with Outlook 2016/2019 connecting via IMAP.
- Navigate to File > Account Settings > Account Settings.
- Select the IMAP account and click Change (or double-click the IMAP account name)
- Enter INBOX in the Root folder path field
- Click Next > Done
All default folder will show up as top-level ones and mail will start populating to the Inbox folder.
Additionally, these settings can be accessed from the Windows Control Panel > Mail. If you do not see Mail, change the "View by" option from Category to Small Items. In Mail, click Email accounts, then double-click on the IMAP account, click More Settings and follow the instructions above.