Why can’t I upgrade from 2019 Freelance Plus to 2021 Freelance Plus at the reduced price?

I have a 2019 Freelance Plus licence, and I want to upgrade to 2021 Freelance Plus. If I go through the “Update” page and select the option to upgrade, click on 2019 Freelance Plus as my starting point and then “Add to cart” at the reduced price ($192), nothing happens.

If I go to my licences page and click on Upgrade, I’m offered the upgrade at the full price ($285).

Anybody know how to upgrade at the reduced price, please?