This known issue was corrected by Microsoft on 2/24/2022.

1. Close Outlook
2. In REGEDIT, browse to HKCU\Software\Microsoft\Office\16.0\Outlook\Profiles
3. Locate the Outlook Profile name of the profile that the shared mailbox is not updating and select it.
4. In Registry Editor choose Edit, Find, and search for registry key name:  0102663e
5. Locate this key in the profile where the parent key is just above the GroupStore as shown below:
   
6. Right click on key name: 0102663e and select Delete.
7. Close Registry Editor and restart Outlook.

It will take a minute or so for the sync to re-establish.

Later versions of Outlook 365 have an “abbreviated” version of the Permissions tab on its folders (like Calendar) that just shows “Custom” without any detail. To restore the proper useful detail in versions 1903 and later:

HKCU\Software\Microsoft\Office\16.0\Outlook\Options\Calendar\
DWORD: ShowLegacySharingUX
Value: 1