From @Skrol29 answer i understand the current situation and wrote a simple function for my case since I need to work with an excel with dozens of merged cells. What my function did is to push all the merged cells located entirely under the placeholder row

function pushMergedCellsDown($TBS, $placeholderRow, $dataCount) {
    if ($dataCount <= 1) {
        return; // No need to move anything if there's no additional data
    }

    $pushDistance = $dataCount - 1;
    $pattern = '/<mergeCell ref="([A-Z]+)(\d+):([A-Z]+)(\d+)"\/>/';

    // Find all merged cells in the XML
    if (preg_match_all($pattern, $TBS->Source, $matches, PREG_SET_ORDER)) {
        foreach ($matches as $match) {
            $colStart = $match[1];
            $rowStart = intval($match[2]);
            $colEnd = $match[3];
            $rowEnd = intval($match[4]);

            // Check if any mergeCell crosses or is on the placeholder row
            if ($rowStart <= $placeholderRow && $rowEnd >= $placeholderRow) {
                throw new Exception("Merge cell crossing placeholder row detected: {$match[0]}");
            }

            // Only process mergeCells entirely below the placeholder row
            if ($rowStart > $placeholderRow) {
                $newRowStart = $rowStart + $pushDistance;
                $newRowEnd = $rowEnd + $pushDistance;
                $newTag = "<mergeCell ref=\"{$colStart}{$newRowStart}:{$colEnd}{$newRowEnd}\"/>";

                $TBS->Source = str_replace($match[0], $newTag, $TBS->Source);
            }
        }
    }
}

the function takes 3 parameters: $TBS the opentbs object, $placeholderRow the row where our data placeholder is located, and $dataCount which is the size of our data.

for my example case, the usage is like this

// Merge data in the first sheet
$TBS->MergeBlock('a,b', $data);

pushMergedCellsDown($TBS, 20, count($data));

Appreciated your work on openTBS library @Skrol29 ^^