I can confirm that I too received my Amex cashback (though only 5%) upon buying an annual season ticket for London Terminals to a Southeastern Railway station located in Zone 3, using the LNER website.
However, I was disappointed that it could only be loaded on to the LNER Smartcard instead of issuing me with a paper ticket like LNER did last year. This was further compounded that the ticket would not load on to the LNER Smartcard at my local train station instead showing Error 57 on the Oyster card reader.
The KGX ticket office could not resolve the error 57 on their system so after dealing with the 2 helpful male staff for over an hour which included trying all of the LNER TVM machines in the main concourse, I walked away with yet another LNER smartcard but this one actually works and can be read by the National Rail Smartcard Manager app on my mobile phone as having a valid ticket. The KGX ticket office no longer have the ability to take pictures to print on the smartcard so I have been advised to use my old photocard even though SE railway no longer require this.
It should be noted that I did have to link the new smartcard with the LNER app before anything showed up on both the SE railway and National Rail apps. This automatically unlinked the non-working smartcard from the annual season ticket.
At the time of writing, even though I bought the ticket online using the LNER website on the 24th January 2024 and the ticket is valid from 31st January 2024, I have still yet to receive my Gold Record Card from LNER.
I wonder who I need to contact to get this sorted out?
BTW, the KGX ticket office don't provide the gold record card as I tried to obtain one last year when attempting to switch over from my annual season paper ticket to the LNER smartcard. The ticket office staff advised me that the reasoning for this is that they don't run any services to a destination within the annual gold card area. Whether or not this goes against what is said on the National Rail website here, I don't know:
https://www.nationalrail.co.uk/tickets-railcards-offers/promotions/annual-gold-card/