Basically, the current behaviour of JIRA is that editing the assignee field will trigger the Issue Updated Event and not the Issue Assigned Event. This is either from editing the assignee field via Edit Screen or Inline editing in Issue view. Some user do not want to receive the notification when the issue is updated, but want to receive notification when the assignee field is updated. If a user use the inline editing or edit the assignee in the Edit screen, the user would not receive notification but the user is expecting a notification.
I think this relates to JRA-26898, since the bulk assigning issue is currently triggering the Issue Updated Event and not Issue Assigned Event.