For resellers, any new license is created with a subscription starting from the date of purchase. We have a separate procedure to make sure renewals don’t loose time like this.
To my knowledge what likely happened here is you got a new account (new license not a renewal) as is often the case when buying from the website or the program.

Open your old license in Notepad and look for your customer number, and compare it to the customer number shown in avast for your new license. Give these to the Avast sales team (sales @ avast.com) and they can likely fix the error for you