I suspect the issue is that the payload is transmitted with some sort of specialized delimiter.
Can anyone from the client or provider application team provide information about what delimiters are used?
As an example, I have seen applications use 0x0B, 0x0C, 0x0E, CRLF, LF, etc. delimiters to denote payload begin and end locations. There is no real standard since this is normally defined by the calling and provider applications as part of their API specification.
If this is a Windows platform, install a tool such as Wireshark on the machine running the recorder. If Linux, you might try using tcpdump.
Start Wireshark or TCP dump and send a request to the recorder.
Stop the Wireshark recording and examine the TCP packets.
The packets can be analyzed to determine if there is a special character used as a delimiter.
If the delimiter is a one byte Hex value, you most likely need to create a custom Delimiter because the "Records are delimited by specific characters" option only accepts values that are in the valid ASCII range (-127 to 127) -- basically, whatever you can key on your keyboard.