|One possible cause is a macro.|
A macro can be written that will fire on the change of a cell or cells, or perhaps when the sheet calculates.
One instruction, such as this, would replace the formula with the results:
Cells(1,1) = Cells(1,1).Value
However, something like this would happen immediately.
Another possibility would be if the formula was highlighted and F9 was pressed.
Sure sounds strange.
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