It's just the latest mistake in SilverRail's implementation of the Routeing Guide to be discovered.
I'm sure SilverRail, as with all other suppliers, will monitor this forum and they will surely take steps to fix the error in IPTIS.
And if they don't spot it, I'm sure it will be reported to them by RDG and/or Greater Anglia anyway.
That's the problem with publishing details of an error or loophole: you may only be able to use it once. In fact I've known people discover a good value fare, post about it, and then never be able to use it, as it's been fixed before they could make the purchase!
If you have an itinerary issued by the retailer who sold you the ticket, the itinerary was issued in conjunction with your ticket at the time of purchase, this must be accepted as a valid itinerary (as stated on NRE and as per contract and consumer laws).