The dashboard "ABCD" is shared for different users with the sharing function of the dashboard. Later, the users of the dashboard are not visible in sections All, Users. 

Below is the screenshot for reference. 

All

The issue occurs as a result of a duplicate user entry when the dashboard has been shared. 

Whenever the dashboard is shared, an entry is placed into the 'pa_dashboards_permissions' table. 
At some point, it may have caused that a specific user has been added twice. Below is the screenshot for reference. 

1. Login to instance and navigate to pa_dashboards_permissions.LIST 

2. Search with the dashboard name = 'ABCD' and you should notice records

3. Identify and remove the duplicate entry for the users.

4. Validate the dashboard again and the user list should get displayed now. 

https://docs.servicenow.com/bundle/orlando-performance-analytics-and-reporting/page/use/dashboards/concept/c_DashboardRoles.html