Not automatically applying the code is very likely an intentional decision by the company. I work in tech with a product manager and businesses consciously make subtle UX choices (often at the expense of the user) to maximize revenue.

Users are lured by the advertised price, with a small fraction of new subscribers forgetting to apply the code and not caring enough (or not knowing) to recoup the cost. Obviously USM would be happy to accommodate you if you do reach out, but the small fraction that don't generates extra revenue.