void exportMyTable(const QTableWidget& table){
    ...
    table.grab().save(fileExportFilePath);
}

void exportMyTable(const QTableWidget& table){
    QTableWidget copyTable;
    copyTable.setRowCount(table.rowCount());
    copyTable.setColumnCount(table.columnCount());
    for (int c = 0; c < table.columnCount(); ++c)
        for (int r = 0; r < table.rowCount(); ++r)
            copyTable.setSpan(r, c, table.rowSpan(r, c), table.columnSpan(r, c));
    ...
}