Edward of Middleham's death date

I keep encountering sources which state definitively that Edward of Middleham died April 9, 1484, which would be a year to the day after Edward IV died. That strikes me as too great a coincidence to be credible (and would certainly have struck the people of that era as a sign that God was displeased with Richard). The Croyland chronicler merely states that he was "seized with illness" in April on "a day not very distant from the anniversary Of King Edward['s death]" and died shortly afterward. Does anyone know where the "fact" that he died on April 9 came from? It isn't Vergil, who is even vaguer about the date (three months after Edward was made Prince of Wales.) Maybe the CC's "not very distant" somehow became the very day, with the distinction between the illness and the death ignored, but who is responsible for obliterating the distinction?

All these constantly repeated "facts" are driving me crazy!

Carol