Experience Cloud Site Users Cannot See Case Email Feed Items on Their Cases

This behavior would indicate that the "Experience Site Case Feed" is not currently enabled in Support Settings.  There are two steps to resolve the described issue.

1. Update the Support Settings:

1. Access Setup.
2. Go to Support Settings.
3. Click Edit.
4. Select the check box for Enable Case Feeds in Experience Cloud Sites.
    
5. Click Save. 

Note: If the option "Enable Case Feeds in Experience Cloud Sites" is not visible in Support Settings, navigate to Setup > Chatter Settings. Ensure that "Enable Actions in the Publisher" is enabled. If not, turn this on and check Support Settings again. The option "Enable Case Feeds in Experience Cloud Sites" should now be visible.  

     ii. Toggle the "Is Externally Visible" flag

1. Navigate to one of the affected email messages.
2. Toggle Is Externally Visible from True to False and Save.
3. Once the save is complete, edit the field and switch back to True.

The email message feed item should now be visible to the Site User when accessing the parent Case record.