A lot of it can be found in support ticket 1538448, but here is the code that build the problematic string that the barcode couldnt encode into a data matrix: 

     private static string Generate2dBarcodeCodeString(List<List<string>> FieldLabelsAndData)
        {

            //assemble the unprintable characters that serve as formatting and record separation. 

            StringBuilder sb = new StringBuilder();
            char rs = (char)0x1E;
            char us = (char)0x1F;
            char gs = (char)0x1D;
            char eot = (char)0x04;


            //assemble the header strings and the recurring strings for formatting. 
            string messageHeader = $"[)>{rs}";
            string formatHeader = $"07{gs}";
            string subDataField01 = "BV";
            string subFieldSeperator = $"{us}";
            string barcodeVersion = "WO.2.0.0";
            string formatTrailer = $"{rs}";
            string messageTrailer = $"{eot}";

            List<string> barcodeFormatAndInformationStrings = new List<string>
            {
                messageHeader,  $"06{gs}", subDataField01, subFieldSeperator, barcodeVersion, formatTrailer
            };

            barcodeFormatAndInformationStrings.ForEach(s => sb.Append(s));

            // assemble the field labels, fields, and open and closing of each section. 
            foreach (var fieldSet in FieldLabelsAndData)
            {
                sb.Append(formatHeader);

                for (int i = 0; i < fieldSet.Count; i++)
                {
                    sb.Append(fieldSet[i]);
                    if (i != FieldLabelsAndData.Count - 1)
                    {
                        sb.Append(subFieldSeperator);
                    }
                }

                sb.Append(formatTrailer);
            }

            sb.Append(messageTrailer);

            return sb.ToString();
        }
    }

The method above was called with the following input:

new List<List<string>> 
            {
                new List<string>{"ts1 ", "test1 ", "ts2 ", "test2 " }
                , new List<string>{"ts3 ", "test3 ", "ts4 ", "test4 " }
            });

And it produced a sizing error when the resulting data matrix's value was set to the output string: