Hello Stephen,
Apologies for the delayed response. Just an update on this thread.
There is an incident that has been posted to the Service Health with ID EX637339 which is related to this behavior.
The user experience of this incident is: Users' email with attachments sent from Outlook for Mac and/or the Outlook mobile app may be delivered without attachments.
More info: While we're focused on remediation, users that have access to Outlook on the web can use this to bypass the problem and successfully send email with attachments. Users may also opt into the Beta Channel release for an immediate solution using the instructions for their operating system found at the following page: https://insider.microsoft365.com/en-us/join/
Current status: As we understand the impact this issue may cause to users, another potential solution that users can implement has been added to the More info section of this communication. Additionally, we expect that this fix will be available, at the earliest, in the August production release due to the complexity of the issue and requisite fixes that need to be implemented. There are several service layers involved that require a fix to address this attachment ID detection conflict, and this will require substantial validation prior to release to ensure the impact to users isn't exacerbated. Additionally, the complexity of the fix and testing prevents us from providing this solution in our weekly update releases. As we approach the month of August, we're continuing to assess a more concrete timeline for when the August production release will be available for users to download.
Admins can view current information and updates on Service health dashboard in the Office 365 admin center (Service health - Microsoft 365 admin center).
For affected customers to easily identify the information here, this thread may be closed. You may subscribe to this thread or refer to Service Health dashboard for updated information. I will also post back here once the issue is resolved on SHD.
Thank you for your kind understanding.
Best Regards,
Ashraf.