I have a 10,000 line spreadsheet - one of the smaller ones - that I created with OPENXML 2.5 SDK.
It has sub-header cells about every 20-25 rows that have a background of grey. Looking at the sheet.xml file, it does show the styleindex listed like it should on each of those cells that should have the grey background.
But after converting to pdf using syncfusion in a VB.net console app - the pdf only has the grey background on those cells for the first page, then no background color for the rest of the spreadsheet.