Using older versions of Outlook for Mac for personal email and calendar
From the thread and the fix I just found, the issues seems related to outlook updates not working with some older OSX system versions.
I was running BigSur 11.2.3 when I upgraded to outlook version 16.69, which caused the issue with attachments. None of the beta versions solved it (currently on 16.71), but after I upgraded to BigSur 11.7, I can see attachments again. Yay!
So, I might suggest making sure you have the most current versions of whatever OSX you are running.