The code is as follows:
  public void formatValue(StringBuffer toAppendTo, Object obj) {
        int start = toAppendTo.length();
        String text = obj.toString();
        if (obj instanceof Boolean) {
            text = text.toUpperCase(Locale.ROOT);
        }
        toAppendTo.append(desc);
        for (int textPo : textPos) {
            int pos = start + textPo;
            toAppendTo.replace(pos, pos + 1, text);
        }
    }

NPOI fixed a bug:

https://github.com/nissl-lab/npoi/commit/c546978db6bb703464ec9c58e8697b3f91709a86

The buggy code of NPOI is as follows:

 public override void FormatValue(StringBuilder toAppendTo, Object obj){...
   for (int i = 0; i < textPos.Length; i++)
            {
                int pos = start + textPos[i];
                //toAppendTo.Replace(pos, pos + 1, text);
                toAppendTo.Remove(pos, 1).Insert(pos, text);
            }
}
The fixed code is as follows:
 public override void FormatValue(StringBuilder toAppendTo, Object obj){...
    toAppendTo.Replace("\0", text);
}