Incorrect calculation of UsedCellRange when conditional formatting is applied to a large cell range.
Workaround:
var usedCellRange = workbook.ActiveWorksheet.GetUsedCellRange(
CellPropertyDefinitions.AllPropertyDefinitions
.Except(
CellPropertyDefinitions.AllPropertyDefinitions.Where(p => p.Name == "DataValidationRule" || p.Name == "ConditionalFormatting")));