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.
My understanding is (from several people here in the office that do not have S/MIME etc. enabled) the problem still exists when receiving emails encoded with SMIME/encrypted etc, because then it remains active and replying or forwarding them affects those attached files.