This problem appeared for me about a month or two ago. It might have occurred when I reverted back to the classic layout of having my calendar, task icon, contacts, etc. appear at the bottom of my list of folders, rather than to the left (which was taking up too much valuable real estate).
I just figured out a solution that worked for me. In Outlook, go to View > View Settings > Columns.
Under the Select Available Columns From dropdown menu, select All Mail Fields. Find the Icon option and Add it. You can then move the Icon option up and down so it appears where you want when looking at your inbox.
Hope this works for you!
