var structured = data
.OrderBy(x => x.RowOrder)
.GroupBy(x => x.RowOrder)
.Select(x => x.OrderBy(x => x.ColOrder).ToList())
.ToList();
ExcelPackage.LicenseContext = LicenseContext.NonCommercial;
using ExcelPackage excel = new ExcelPackage();
var workSheet = excel.Workbook.Worksheets.Add("Sheet1");
workSheet.Cells["A1"].LoadFromCollection(structured, true); //Здесь вылетает
workSheet.Cells.AutoFitColumns();
using var memoryStream = new MemoryStream();
excel.SaveAs(memoryStream);
var reportData = memoryStream.ToArray();