If you can upload the original photo that might help. I suspect you have some artifacts in your photo that are coming out in the very fine details of the engrave. Sometimes you can spot them if you start messing with the curves. This sort of problem often happens when you’re working with a “picture of a picture”, like a camera pic of an existing photo print or even a scan of a print.
Another possibility that comes to mind is that the dithering pattern that glowforge uses when converting to dots is making a visible pattern.
Hmm it’s draftboard, which is pretty uniform stuff, so it’s not likely to be the material itself, though it may be. Can you reproduce the same pattern if you flip it over and engrave the back? (that way you’re not wasting more materials to test it). if the pattern is identical on the other side then we know it’s very unlikely to be variations in the material. If, however, the pattern of defects is a mirror image of the defects on the other side then it starts to seem like there are “stripes” of weirdness in that piece of draftboard. Generally Glowforge refunds defective proofgrade materials, so it might be worth the test.
Are you engraving on masking, or did you remove the masking prior to engraving? The masking might have some variation.
Beyond that, there are some best practices to follow when prepping a photo, check out #9:
You may already know all that stuff, but it never hurts to overshare 
Anyway, upload the original photo and we might be able to troubleshoot the source art and rule that out at least.