This will only work when the task is created by the form submitter instead of Asana. (check the first entry in the All activity log in the task details)
To fix this, go to your form settings and set the form access to Organization only AND select at least one of the three blue switches below, under Replies.
That way, the Email question in the form will switch to disabled because it will automatically collect the user’s email when they are logged into Asana on their browser and use the form.
Asana will use this to set them as the task creator so your rule will then work with that variable.