Mate Tea turning dark green

I’m mostly just into black tea and green tea, but recently had some really nice cups of mate. Soon after, I bought my first pack of dry mate leaves from a local store, so I can cook it anytime at home.

The taste is great, the leaves are looking fresh, but I have one question: if After infusion I take all the leaves out and drink it from a usual mug. The tea has a light green yellowish color, but if it stays in the mug for +30-40 minutes it gets really dark, if it stays there for more than 2-3 hours it gets darker than black tea. So far I wasn’t able to find an explanation for this, does anyone know why this happens? Is it normal or have I bought a spoiled leaf?