Here is sample code, which copy first row of table and populate it with some text


    public static void sample(XWPFTable table) {
        final CTRow templateRow = (CTRow) table.getRow(0).getCtRow().copy();
        final XWPFTableRow tableRow = new XWPFTableRow((CTRow) templateRow.copy(), table);
        tableRow.getCell(0).setText("Sample text"); // OK
        table.addRow(tableRow); // <-- cloned CTRow is copying
        tableRow.getCell(0).setText("Another sample text"); // <-- Modifying old CTRow  
        // After writing document row still contain "Sample text"
    }

The problem is in method XWPFTable.addRow


   public void addRow(XWPFTableRow row) {
        ctTbl.addNewTr();
        ctTbl.setTrArray(getNumberOfRows() - 1, row.getCtRow()); // <-- copy content of row CTRow
        tableRows.add(row); // <-- XWPFTableRow still points to own row.getCtRow() and subsequent calls not affected final document
    }