When an assignment is marked with feedback an email is sent to the student telling them that feedback has been left and a link to it.
If this feedback is then altered or added to no email is sent to the student informing them of this.
This is important as the same assignment is often used for re-submissions of work for second marking.
There is a forum discussion for this here:
There is also a closed ticket:
However I think this ticket might have been closed in error