This can happen sometimes if the upsell product has had any changes to variants it e.g. changed from fixed price to dynamic pricing, new variants added or old ones deleted.
Here's how to fix it in 2 minutes:
Go into the cart and navigate into the One Tick Upsell block or Checkout Button block settings (wherever your auto-fulfil has been set up).
Remove the selected product from the upsell by clicking the x button.
Then search the product again and re-add it and click save.
This refreshes the product and auto-fulfil will work again.