Fix#641 (marked stale in 2018, but now reopened). When a sheet's title is changed, PhpSpreadsheet updates references to the old sheet name found in formulas. Which is a good idea when the sheet is attached to the spreadsheet, but a bad idea when it isn't (often because it has been cloned without re-attaching to the spreadsheet). This PR continues to change formulas in the former case, but will no longer do so for the latter.
oleibman