That looks like it might just be out of focus. It’s not drifting, as the top and right edges are very sharp.
The little spines along the bottom of the engraving appear to be following the darker grain lines, so that supports that idea - diode lasers use visible light, and are absorbed better by dark colors, so you’re getting a darker burn where the grain lines are.
Most diode lasers have a knob at the bottom of the laser module that you twist to focus. Use the ‘Fire’ button in LightBurn to turn on the laser at the lowest power you can reliably see it, point it at something dark, and twist the lens until you get the smallest possible spot. Most higher power diodes are a rectangle, not a dot, so don’t worry if that’s what you see.