Shopify's Cart API returns 200 but completely ignores contents

[CameronKaiser]

It's simple: I make a clean API call to cart/change.js, Shopify recognizes it with a 200, but then just returns the cart without having done anything to it. There are 24 units of this SKU in Shopify's inventory, so it's not some maximum units issue. Here is the sequence of events:

I hit it with a payload to set the cart item ending in 394 to 8 units, receive a 200 back, but then it just sends me the cart with the previous amount of units, 5. It just won't update. This doesn't make any sense to me, does anyone know what's up?