I have now looked at this in detail. Conclusion is that the formula parser in PHPExcel_Writer_Excel5 is at fault.
The unary minus operator is written incorrectly. Sample formula that is not working with PHPExcel_Writer_Excel5:
Although the formula looks correct when the generated xls file is opened in Excel, it is not ok internally. (But double-click or enter can rectify the problem.) Effectively, the unary minus token is put around the whole expression
which is not the same as the previous formula.
Workaround until this bug is fixed:
Put parenthesis around -A1 as follows when you write formulas using PHPExcel_Writer_Excel5
@daklan: For example, you have formulas that look like this:
If you change to this I think it will be ok: