This is a known issue with Outlook 2016/2019 connecting via IMAP.

1. Navigate to File > Account Settings > Account Settings.
2. Select the IMAP account and click Change (or double-click the IMAP account name)
3. Enter INBOX in the Root folder path field
   
4. 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.

Tags: Email, Outlook