Hi Kevin,
To my knowledge, Banners will only appear for 5 seconds and then disappear on their own. Also, if multiple emails arrive at the same time, Outlook will only display one banner. While Alerts won't disappear on their own, you must dismiss them. And if multiple
emails arrive, each incoming email will appear an alert. For your reference, see
Outlook for Mac for Office 365 New Email Notification Options. In this scenario, we suggest you select Alerts as the notification style to check if it makes any difference.
And if the issue persists, it could be a client issue. To better assist you, you can try to contact the Outlook for Mac in-app support team via
Help > Contact Support. The team focuses on Outlook for Mac client issues with rich experience. We sincerely appreciate for your understanding.
Regards,
Marvin