IOSv8.4 displays hidden/protected cells in Excel worksheets

IOS 8.4 opens all hidden and protected cells in Excel worksheets.

We have created a secure Excel document and password protected the sheet, hiding formulae and sensitive data.

When emailing this Excel spreadsheet, anyone on an iPhone/iPad can see the entire document, including protected data.

Android phones display the spreadsheet correctly (Hidden and protected cells are NOT visible)

This is a massive security issue for us, as we do not want the recipients to see the hidden data. We have no control over what device they will use to open the document.

I have seen discussions on this issue in the past, and there were suggestions that the user should uninstall an app on their iPhone - this is not a solution for us as we are not using an App to open the Excel sheet.

Any advice on what we need to do? Thanks