The code is as follows:

public static byte[] getBytesInCodePage(final String string, final int codepage) 
    throws UnsupportedEncodingException 
    {
        String encoding = codepageToEncoding(codepage);
        return string.getBytes(encoding);
    }

When codepageToEncoding throws exceptions, encoding will not be set. 
NPOI fixed a bug:
https://github.com/nissl-lab/npoi/commit/9ee6fa7de8361239dc962ccf9d5c99e65587b234

The buggy code is identical, but the fixed code handles exceptions:

public static byte[] GetBytesInCodePage(String string1, int codepage)
        {
   
            String cp = CodepageToEncoding(codepage);
            Encoding encoding;
            try
            {
                encoding = Encoding.GetEncoding(cp);
            }
            catch (Exception)
            {
                encoding = Encoding.ASCII;
            }
            return encoding.GetBytes(string1);
            //return string1.GetBytes(encoding);
        }