個人用のメールと予定表に以前のバージョンの Outlook for Mac を使用する
Hi! Sorry to hear that you have trouble with this situation. As mentioned in the next link, this is a known issue and is currently being investigated by Microsoft. You can try the workaround mentioned there until more information is available:
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