Office 365 Outlook Message Windows don't close on successful send

To reproduce:

Login to outlook.office.com
Compose a new email
Click the pop out button to open a new window to continue composing.
Send the message.

What happens - the Sending animations run, then stop once the message is sent (it does send the message so this isn’t a show stopping bug). In Chrome the window closes. In Brave it stays open. This is the same whether the shields are active for office.com or not.