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.