As Zax mentioned, I think that might be because the grain structure of the Bamboo is causing the surface to burn at different rates. If the bands are still horizontal when engraved diagonally then it's likely the grain, if they follow the burn axis then it's more likely a hardware issue. As a trial have you tried a very plain (and cheap) wood like Lime or Bass wood? It's used a lot for carving so is available in some woody craft stores or on line.
Just a thought..