Further detail:
This is an isolated event involving just 1-2 JIRA users.
One user claims he previously received notification emails when an issue was assigned to him for approval. But for the past few weeks, those email notifications have stopped.
He's an active user (obviously), he receives notification emails when he is mentioned/tagged in a comment.
I have checked every setting I can think of. I made sure there is a notification scheme associated with the project. When I use the Notification Helper, it reports the user "receives notifications for the 'Issue Assigned' event."
What else could be the issue. No other users report any problems receiving notifications.