Which website are you reading?Nothing in terms and conditions to say it does not apply to season tickets.
Put in a query with topcashback as purchase did not track
My source is here - https://www.topcashback.co.uk/chiltern-railways/
And under "Terms & Conditions and other useful information", then "What will stop me getting cashback?" it states
In any case there are legitimate ways of getting a much larger rebate on season ticket purchases than a cashback site where some of the retailers play silly games with the payouts!If you purchase a season ticket.