This is a known limitation of the "New Outlook" feature in Outlook for Mac. This and other known issues are documented here: https://support.microsoft.com/en-us/office/the-new-outlook-for-mac-6283be54-e74d-434e-babb-b70cefc77439
Microsoft has not resolved this issue at this time, and there are no work-arounds.
If you need to view a shared calendar or mailbox, please turn off "New Outlook" in the upper-right corner of the Outlook Desktop app window.
For assistance, contact the Service Desk.