Hi @Scott R ,
Welcome to our forum!
Based on my searches, someone recently reported this issue to the Apple forums, which seems to be an issue with macOS Big Sur 11.6.6, as he reproduced the bug on a fresh install of 11.6.6 before installing any third party.
See: https://discussions.apple.com/thread/253907764?answerId=257335259022#257335259022(Note: Since the web site is not hosted by Microsoft, the link may change without notice. Microsoft does not guarantee the accuracy of this information.)
Given this case, you can refer to the workaround in the discussion: Save the attachment to the Desktop (or a folder on the Desktop), and open it from there.
In addition, the Outlook for Mac team is currently investigating the issue with Apple and I will update in this thread when more information becomes available.
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