This is not actual bug report, but I'd like to help solving issue.
Sometimes I get black rectangles (kind of "lost tiles" during rendering) on final exported image. When they exist on final image, they also do on zoomed preview inside Natron viewport. Sometimes I was getting tiles that has some weird colors (like only yellow or only green). I will paste screen if get this again. In the meantime I can help tracking this down, but need guide here how to do it, I mean how to get which node is causing this.