Strange "lines" when engraving

What could cause this? Looks like there is some constant lines at exactly same distance apart from each other. Happens with engraving and it’s not so visible in wood but in acrylic it’s kind of looking bad.

Engraved with beambox pro. 500dpi, 150mm/s, 18% power. Medium clear acrylic, not sure if XT or CS.

Maybe try a lower dpi. It might be trying to squeeze in another row / pass where it has already engraved?? IDK just guessing.

I’ve tried with different setting and it’s most visible with 500DPI. Changing engraving speed doesn’t seem to make any difference. With 250DPI the lines are more apart from each other and with 1000DPI they are not visible. I still think this is not how the machine should work so switching to use 1000DPI is not an option. Also it makes the process very time consuming.

Here’s another photo of my tests. Has anyone else experienced something like this?