Hi there,
@anxiouspenguin , I am having the exact same use case as you, tried all the things you tried and am running into the same problem. Funny your post is only 3 days old!
Have you been able to fix it already? The problem still persists for me unfortunately.
I’m assuming you also used these docs here: https://shopify.dev/docs/storefronts/headless/building-with-the-storefront-api/cart/manage#step-3-increase-an-items-quantity
There it’s clearly stated to do it this way, and if something isn’t correct about this way of doing it, then Shopify should urgently update their documentation!
Regards,
Silvan