If the selling_plan is not being passed in the payload in the new version of Dawn (v8.0), it can cause issues with registering subscription products in the cart. Here are a few potential solutions:
Make sure your subscription products are set up correctly: Double-check that your subscription products are set up correctly in your Shopify admin. This includes making sure the product has a selling plan attached, and that the selling plan is configured correctly.
Check your theme code: If the selling plan is not being passed in the payload, it's possible that there is an issue with the theme code. You can try checking the cart.liquid and cart-item.liquid templates to see if the selling plan is being referenced correctly.
Update your theme code: If you've confirmed that the issue is with the theme code, you can try updating the code to include the selling plan in the payload. Here's an example of how to do this:
In your theme code, find the form element for the cart, which will look something like this: