The header you asked Twilio to stamp was omitted because it contains illegal characters in its value. Including this header would have resulting in Twilio generating a message that runs contrary to the SIP specification.
If you specified headers in a SIP URI, invalid characters were most likely introduced by an escaped character. Escaped characters in SIP URIs are defined by a percent sign (%) followed by two hexadecimal digits specifying a character code.
If you are generating SIP headers from user input, ensure your application properly accounts for non-alphanumeric characters such as punctuation and whitespace, as well as international characters.