(completely irrelevant post follows. I missed the gaping holes in the engrave!!)
It’s an unfortunate artifact of the engrave process when using the “vary power” option, there’s nothing wrong the file or settings. It also shows up most on acrylic. Take a look at the deep engraved sections in the close-up of this little piece I made last night. There’s simply no way to avoid them, it’s a result of how the files are converted for printing.
You can, however, correct them post-engrave with a flame from a small torch, but that takes practice and would damage the surrounding un-engraved area in your design.
You can also run another pass at a different focus level to “soften” the engrave, but that also takes a lot of experimentation and with the more recent “auto focus”, seems incredibly difficult to do with any consistency.