The CellPropertyChanged event is raised only when some value in the data structures holding all spreadsheet values changes. When you modify a value in a cell that is referred by a formula actually the data stored in the cell holding the formula stays unmodified. Only the result value of the formula is invalidated and is calculated the next time the result value is accessed. That is why CellPropertyChanged event is not raised for the cell containing the formula.
I have attached a sample project showing how to track if a CellFormulaValue is invalidated.
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