When Outlook is being closed the program will run in the background syncing up with the email service.  Doing this insures your email will be up to date and running as expected.  However, sometimes when you go to open it again, it does not.  That is because it is running in the background, syncing.

The best way to handle Outlook, (2019, in this case), is to open it and leave it open until you are finished with it for the day.  Opening and closing Outlook multiple times during the day will cause this issue to re-occur.