It is a valid ticket since it has been issued by an accredited retailer; TfL have given RDG the authority to issue tickets on their behalf, and in turn RDG have licensed Avanti and their TIS (Ticket Issuing System: Pico4UK) as an accredited retailer.
As
@bcarmicle says, I would put money on TfL RPIs wrongly claiming it is invalid, but you would have to be pretty unlucky to come across them so I would be prepared to deal with that situation if it arises.
In terms of what has happened, the flow (i.e. origin/destination pair and ticket type) must have been accidentally enabled for eTicket fulfilment in the RCS (Retail Control System) by Greater Anglia, who are the fare setter. I am sure this will be fixed fairly soon now that you have posted about it on the forum.
However, Pico should probably also have a hardcoded rule that prevents the issuance of eTickets for journeys involving the Tube, even when eTickets are enabled for the tickets in question. So although they were just following what's in the industry data feeds, other TISes would not have offered an eTicket for this journey because they know that it is only liable to cause passengers difficulties.