Apologies for posting this here, I coudnt find the appropriate GitHub repo. Will gladly post it there as well if someone can point me in the right direction!
Using this script (https://sdks.shopifycdn.com/buy-button/latest/buy-button-storefront.min.js) to build a custom website front end has an issue with quantity. If you add a product to the cart, and then press the minus button to set the quantity to zero, I would expect the product to be removed from the cart. It does not get removed and then the quantity buttons no longer work as the cart thinks the product is gone from the cart in the background but it is still there visually to the user. This results in console errors when pressing the buttons after setting the quantity to zero.
This is on a site thats been active for sometime and i think its a recent bug, as i dont remember this happening previously.