That is not good. I've bought 4 frames from Chris but not that one. Strange clamping system, especially 4nm. Did you use a torque wrench?
I'd email him and see what he says. AFAIK, frames that are cracked at the seat post are written off. Can it be just a paint thing? If you look inside the tube what do you see? Can you film what happens when you sit on the frame? Paint crack, fine, frame crack, landfill?