[randomswede]

As a possible hypothesis test, next time, could you leave the new opt-in in place for a month before unsubscribing?

If that reliably then has a new list added exactly one year later, it is a strong signal that it indeed based on you unsubscribing. But, if it instead happens at 11 months, it is a string signal that new lists are added at a specific month/day.

I mean, I would not be surprised if it happens one year later, due to Elsevier tracking when you unsubscribed, but this is a testable hypothesis, so testing it would be a Good Thing.