Using older versions of Outlook for Mac for personal email and calendar
as far as I could test, and from what I see on the other posts, it is only for SMIME (signed/encrypted) emails.
If you do not use one of the 2 you should be fine. Or at least this is what the behavior indicates so far.
right now I am on 16.66 and so far it looks good. I have only send about 10 emails but at least so far all of them had the attachments.