Interesting…I wonder if this might be a factor, especially considering your test piece at different pitches is clean. Is there any way you could force the CAM to engrave one leaf at a time, raster style, rather than jumping around and coming back ? At such a small pitch, tiny mechanical errors (backlash, slop, …) might introduce a tiny offset between one line and the next one engraved when the DDB comes back from somewhere else. It would be interesting to visualize the G-code, and see if the transition areas you are seeing match the rapids between.
Depends on your CAD file, you may be able to just select one leaf and apply the toolpath to that. I did not want to ask for the full G-code file or the full design file as I guess you would like to protect your artwork.