I agree it would be good to get this sorted, though I think it’s unlikely I’ll have the enthusiasm to go much further than seeing what TfL say. Although I have some interest in trains and the Underground in particular, my interest in the complex world of ticketing extends only so far as minimising cost while maximising convenience.
I should have said earlier that there are three possible causes:
1. the ticket itself is not valid for unlimited travel in z1-9
2. the ticket is not being coded correctly on the smartcard
3. the gates and POMs are not programmed correctly
I’ve concluded that the cause is no.1 because I can’t find any evidence to the contrary; that is, where it’s described as being for unlimited travel in z1-9 or as a z1-9 Travelcard. Even GA’s reply to my enquiry can be interpreted in various ways as it’s obvious that a ticket from Amersham to London must in some way be “valid for travel in London Fare Zones 7-9”.
https://www.brfares.com/!fares?orig=AMR&dest=MYB&rlc=SRN says only “Valid for unlimited travel in London Zones 1–6”
NRE uses the same wording as GA’s app: “Also valid for travel in London Fare Zones 7-9”, and explains code ODT as “…unlimited travel throughout London on National Rail, London Underground, DLR, London Trams and London Bus services within Zones 1-6.” (Poorly worded, as it’s valid on TfL buses no matter where they are, and it fails to mention the HEx & HS1 exclusions.)
https://www.greateranglia.co.uk/travelcards is more comprehensive, even saying “They are issued to cover Zones 1-6 only” and “One Day Travelcards are valid for one return journey from the station of origin to London fare zones 1-6, and for unlimited travel within the zones.”
Clear as mud, then. I’ll report back when TfL reply.