Caller and Assigned to fields are missing on forms for tables extended from Task
The user is likely failing an ACL on 'sys_user.name' which restricts them from seeing fields such as 'assigned_to' and 'caller' that render 'sys_user.name' as a value. (PFA)
Reconfigure the ACLs on 'sys_user.name' to allow proper access to these records for your agents per your business requirements.
You may need to clear the instance cache for the changes to take affect.