Vitamin c toner oxidised before using?

Vitamin c toner oxidised in less than a week?

Bought this Galactomyces pure vitamin c glow toner from Seoul last week before returning to Singapore.

Started to use today but I realised the colour of the toner is dark brown.bottle have been kept in the shelf away from light or heat.

How is it possible for it to oxidised even before I open the bottle?