Formulas Not Calculating on spreadsheet opening in MS Office 365 Build 1803

[ghost]

Since we've upgraded to MS Office 365 Build 1803, no formulas are calculating when the spreadsheet first opens, if the spreadsheet is generated by PHPExcel.

I've reported the issue to MS, but I'm not sure if anything can be done on the code library side?

Multiple people have this issue - I've commented in both threads I'm pasting below:
https://answers.microsoft.com/en-us/msoffice/forum/msoffice_excel-mso_other-mso_o365b/excel-2007-files-downloaded-from-intranet-cells-do/723f4e27-a4f9-493c-a538-11e2bc27d29b?rtAction=1523970193877

https://techcommunity.microsoft.com/t5/Formulas-and-Functions/Build-1083-formulas-not-working/m-p/183431/highlight/false#M4791