National Rail Enquiries and the C2C journey planner both think that the London-St Albans City offpeak day return at £12.30 is valid for Euston-Watford Junction-St Albans Abbey-walk to St Albans City
Yes, they do, because the company who provides their fares data has an incorrect implementation (it's possible someone may read this and get it fixed, and it will then no longer show as valid).
If anyone wants to book a ticket with an accompanying itinerary, this would be evidence of a contract, and the ticket would therefore be considered valid, however do not be surprised if it's
not a stress-free journey.
The correct action of staff at St Albans City would be to either make notes of the error and report it, or to withdraw the ticket and issue a free of charge replacement ticket, and at all times to be respectful and helpful towards the customer.
However in practice the customer may (incorrectly) be sent back to St Albans Abbey and may be mistreated.
I've experienced some good staff, and some bad, at this location, so it is difficult to predict the outcome. It really depends on what staff are on duty and how they react.