I always had an issue with saving large amounts of data (e.g. 10K or entries), but they all ended with 504 Gateway Timeout error. I updated the library to the latest and now I'm getting this when I try to save such documents (small ones are saved without any issues):
Next PhpOffice\PhpSpreadsheet\Calculation\Exception: Worksheet!Z3280 -> Formula Error: Unexpected operator '=' in /vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Cell/Cell.php:563
Stack trace:
#0 /vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php(1564): PhpOffice\PhpSpreadsheet\Cell\Cell->getCalculatedValue()
#1 /vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php(1729): PhpOffice\PhpSpreadsheet\Writer\Xlsx\Worksheet->writeCellFormula()
#2 /vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php(1487): PhpOffice\PhpSpreadsheet\Writer\Xlsx\Worksheet->writeCell()
#3 /vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xlsx/Worksheet.php(101): PhpOffice\PhpSpreadsheet\Writer\Xlsx\Worksheet->writeSheetData()
#4 /vendor/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Writer/Xlsx.php(398): PhpOffice\PhpSpreadsheet\Writer\Xlsx\Worksheet->writeWorksheet()
#5 PhpOffice\PhpSpreadsheet\Writer\Xlsx->save()