It seems that currently the Issue Assigned event is only fired when

• The Assign Issue Operation is used to assign an issue
• The Issue Assigned event is explicitly called in a workflow on a transition using a post-function (where the assignee may or may not have been modified)

It would be nice to have both the current and new assignee notified even if the issue were changed when the issue was edited, or on an arbitrary workflow transition where the assignee field as available.