The 999-X231.pdf on page 33, describes the CTX Business Unit: This contains the value from the business unit identifier specified in CTX01-1. This element is defined as type AN and min/max of 1/35.
For the 270/271, the TRN02 is the business unit for use in the CTX01.01, so the value of the TRN02 would be provided in the CTX01.02. However, the TRN02 in the 270/271 is a type AN and min/max of 1/50. This is causing the 999 to not be valid when the TRN02 contains a value greater than 35.
Question; Should the value be truncated?
Yes, the value must be truncated or the 999 will not be syntactically valid. In reviewing this RFI, X12C will file a work request to increase the data length of DE 9998 Context Reference.