Line through lettering

There are several lines, none of which extend across the entire image. In general, they affect one side of a character, not the other side a few millimeters away, and exactly follow the wood grain.

I think you’re looking at natural variations in the wood, where some parts of the grain simply don’t react to the laser power level in use.

For a quick check, engrave the same pattern with the same wood placed at an angle on the platform. If the unburned parts are aligned with the wood, rather than the laser scanning path, it’s the wood.