I found the same thing trying to book DFW-RDU last week.
Not sure exactly how it used to work, but 173/174 used to either originate in/continue to DFW, or would run DFW-RDU-LGW-DFW/DFW-LGW-RDU-DFW. Or something like that.
From this
very helpful thread from
777Brit, it appears that 173 now turns right back into 174, flying RDU-LGW-RDU.