Ascorbic acid is another word for vitamin C, but can't be put as Vitamin C on the label (EU regulation). Ascorbic acid is an antioxidant used as an additive to prevent oxidation (browning) mainly of apple juice (but also pear and carrot juice).
We put up to 0.2% ascorbic acid in some of our juices. This varies per type of fruit and time of year. It disappears during the shelf life. The ascorbic acid level does indeed decline during the shelf life. It oxidises with oxygen. That means that when browning is visible, there's no more ascorbic acid left.