TIL Japan has something called アメリカンコーヒー, often referred to in English by the unintentionally condescending name "weak coffee".

This was semi-debunked by James Hoffman semi-recently.

Technically yes lighter roast has more caffeine in the beans because less is burned off.

However, you use more darker roast because lighter roast is heavier (because darker is effectively more dry). Also because the darker roast is more roasted, the caffeine in the beans may be more easily accessible/dissolvable.

I think roughly it evened out in the end.