Editing SUBTOTAL causes it to convert to text only

I have a subtotal formula which works fine. I can copy it from one column to another and it still works properly. But if I directly edit it in any way -- change the text prefixed, change the range by direct edit or expanding the box -- it stops working by becoming text only. If I use Replace to change the prefix text or range, it continues to work correctly.
Auto-calc is on; F9 does no different. Using Excel 2007, activated, format as general or text, xlsx or xlsm -- no difference.