Vendor/phpspreadsheet/phpoffice/phpspreadsheet/src/PhpSpreadsheet/Reader/Xls.php
private function readMergedCells()
    {
        $length = self::getUInt2d($this->data, $this->pos + 2);
        $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);

        // move stream pointer to next record
        $this->pos += 4 + $length;

        if ($this->version == self::XLS_BIFF8 && !$this->readDataOnly) {
            $cellRangeAddressList = $this->readBIFF8CellRangeAddressList($recordData);
            foreach ($cellRangeAddressList['cellRangeAddresses'] as $cellRangeAddress) {
                if ((strpos($cellRangeAddress, ':') !== false) &&
                    ($this->includeCellRangeFiltered($cellRangeAddress))) {
                    $this->phpSheet->mergeCells($cellRangeAddress);
                }
            }
        }
    }
private function readMergedCells()
    {
        $length = self::getUInt2d($this->data, $this->pos + 2);
        $recordData = $this->readRecordData($this->data, $this->pos + 4, $length);

        // move stream pointer to next record
        $this->pos += 4 + $length;

        if ($this->version == self::XLS_BIFF8 && !$this->readDataOnly) {
            $cellRangeAddressList = $this->readBIFF8CellRangeAddressList($recordData);
            foreach ($cellRangeAddressList['cellRangeAddresses'] as $cellRangeAddress) {
                if ((strpos($cellRangeAddress, ':') !== false) &&
                    ($this->includeCellRangeFiltered($cellRangeAddress))) {
                    $this->phpSheet->mergeCells($cellRangeAddress);
                }
            }
        }
    }
MERGEDCELLS.

This record contains the addresses of merged cell ranges
in the current sheet.

--    "OpenOffice.org's Documentation of the Microsoft
Excel File Format"