Microsoft has shared the following information regarding this issue:
We've confirmed that the settings change deployment has finished. Affected users will need to refresh their Outlook on the web browser page to enable these settings, which will cause the client to automatically reconnect to the server at five-minute intervals. Once this initial refresh is performed, users having this problem will see new email notifications within a maximum of five minutes after the message arrival.
If you are still seeing issues with Outlook on the Web not automatically refreshing, please notify the Help Center at x58888 or firstname.lastname@example.org
. Thank you for your patience!