Vitamin C is a temperamental thing. It oxidizes very easily, and then just becomes this inert thing. Luckily, it turns color when it oxidizes, so it lets you know it happened. Skin care products try to stabilize it with other ingredients while packaging it in UV protective containers. Some even go so far as to recommend keeping the product refrigerated. There are now many options for adding Vitamin C with the majority of them reducing the effectiveness of the Vitamin C to gain stability in the product as in some are not as efficiently absorbed by the body, but won't oxidize. Some are water soluble, some are oil soluble. Of all of the ingredients, VitC is definitely the most finky

> Vitamin C is a temperamental thing. It oxidizes very easily, and then just becomes this inert thing.

Doing exactly what it was designed to do, biologically speaking.

as far as vitamin and mineral intakes, vitamin C is the thing you need to worry about the least because it's so highly abundant in so many fruits and vegetables. I've calculated VitC density in foods and normalized it to calorie intake: It's incredibly abundant in many real foods.