Using older versions of Outlook for Mac for personal email and calendar
I've given up on direct updates from Microsoft, and now use this as the best source of information (credit to the other person who posted this earlier in the thread): https://support.nhs.net/2023/07/microsoft-365-alert-service-degradation-exchange-online-users-email-with-attachments-sent-from-outlook-for-mac-and-the-outlook-mobile-app-may-be-delivered-without-attachments/
Latest updated from yesterday 21st September: Microsoft deployed a fix to update attachment logic that they have confirmed will reduce instances of this issue from occurring when users frequently add and remove attachments by 40 percent. Their development of a long-term solution to this problem is contingent on the telemetry they are collecting subsequent to the delivery of their attachment logic update, and Microsoft will provide a better estimation on the development of the long-term fix when available.