Using older versions of Outlook for Mac for personal email and calendar
You can get the latest info on this issue from the Support Bulletin at
https://support.microsoft.com/en-us/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
There are also multiple other threads on this problem with some workaround suggestions. e.g.
And, as Marla suggested, submit a Problem Report so Microsoft knows how many people this is impacting. Every Report moves the issue one notch higher on the fix list.