It's worth noting that shiny surfaces don't radiate heat very well. You can see it with an infrared temp gun too. Two surfaces, one shiny and one matte that measure the same temperature with a contact thermometer will read wildly differently with an infrared temp gun or a thermal camera.
That's probably the problem Noctua is having, shiny aluminum heatsink fins don't radiate heat for the camera to see. They conduct it inside themselves nicely and transfer it to air nicely via convection, but the camera can't see that.