I am not sure whether this is a bug, but shouldn't the system message generated when a user deletes his/her post also be normalised if the user requests deletion?
Case in point (viewable for 10K and diamonds on Math.SE): http://math.stackexchange.com/a/44378/1543
The user/OP's account was deleted, as one can see from the greyed-out normalised username attached to the post. But the system message indicating deletion still shows the user's original username.
Ignore the "edited by" line, some other user edited the post before it was deleted. But look at the bottom left where it says "deleted by C" (I left the first letter to show that it is not "user9413"; it is also not one of the diamond mods); the displayed name there is/was the username of the now deleted user9413.