Hi @Beth Olson ,
According to my research, which is an expected behavior. To work around, you can follow the steps:
1.Sign in to your account in Outlook on the web.
2.On the Outlook on the web navigation bar, click your name. A list appears.
3.Click Open another mailbox.
4.Type the email address of the other mailbox that you want to open, and then click Open. Another Outlook on the web session opens in a separate window, allowing access to the other mailbox.
More information in this link for your reference: Open and use a shared mailbox in Outlook on the web
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