Summary

 Unable to view/add a user to the assignee field of issues in Jira Cloud projects, in spite of all the permissions in place. 

Diagnosis

 The issue was due to an indexing/synching issue in Jira

Cause

Indexing/synching issue in Jira

Solution

Remove the user from the Jira software default group and re-add to it, which triggers a sync, which resolves the issue.

Below are the steps to follow:

1. Navigate to the user management page.
2. Identify the product Jira software default group.
3. Search for the user under the Users section.
4. Remove the user from the default group 
5. 
6. Re-add the user to the default group.
7. 
8. This process will trigger a sync which resolves the issue.