I used to think the same, that roasting burns off the caffeine so lighter roast = more caffeine.

But when I looked for a source on that one day it seems like that's a myth. There can be differences based on whether you measure by weight vs volume but that's about it. https://www.kickinghorsecoffee.com/en/blog/caffeine-myths-da...

So now I just say I like light roasts because I like to taste the coffee and not the roast.