Hi Mark,
Based on your descriptions, the issue occurred after upgrading from Outlook 2011 to the recently released Outlook for Mac.
Given the situation, it should be more related to the Outlook client itself. Since this forum mainly focuses on the Office 365 online services, it’s suggested that you raise the issue in the Outlook for Mac forum (it's the recently published support channel after the Outlook for Mac was released) for proficient assistance.
Thanks,
David
