Build 1803 formulas not working

I make an Excel speadsheet using PHPExcel for a client that has never had any issues until Office 365 Excel Version 1803 came along.  The client downloads the spreadsheet from my website, opens it and sees that all the formulas that used to display the correct values now all show 0.  The Ctrl Shift Alt F9 keyboard combination successfully refreshes all the formulas but this should be unnecessary and is not required in any other version of Excel I have tested.  The client has a user still on Version 1802 which doesn't suffer this issue.

Some info:

Protected View for files download from the internet has been disabled

I have checked that Calculate is set to automatic

All fields have been set to number fields

I have tested a spreadsheet with just a single formula:  A1 = 3, B1 = 2 and C1 =A1*B1 and I still get a value of 0 in C1 until I do the Ctrl Shift Alt F9 sequence.

Does anyone have any ideas of what else I could try or is this a bug in Version 1803? I'm suspecting a bug as the spreadsheet has worked fine for the last 5 years until this build but who knows, there could be a checkbox somewhere that now needs unchecking.

*****************Edit 18th April*************************

This issue is now being looked into by Microsoft personnel over at the link posted below by Mick Tindall.  Check there for an update to this issue. Hopefully a fix will be found soon.

*****************Edit 19th April*************************

The link mentioned above now contains a fix on page 3.  

You just need to add $objWriter->writeAttribute('forceFullCalc', ($recalcRequired) ? 0 : 1); to the _writeCalcPr function in your_PHPExcel_directory_name/Classes/PHPExcel/Writer/Excel2007/Workbook.php file directly under $objWriter->writeAttribute('fullCalcOnLoad', ($recalcRequired) ? 0 : 1); line which is line 204 in PHPExcel-1.81 and PHPExcel-1.8.0