Using older versions of Outlook for Mac for personal email and calendar
This is a confirmed bug. Please see Microsoft's communication here: https://support.microsoft.com/topic/replying-to-or-forwarding-an-email-does-not-include-the-original-message-in-the-email-body-in-legacy-outlook-for-mac-2aff3768-e7c5-4f5e-90ab-2e3976f424e8
Current suggested workaround: From the Outlook menu, uncheck the Legacy Outlook or select the toggtle button at the top o the Outlook for Mac window