In this image of my render, the top and bottom layers are unclosed, unfilled lines/paths (like circuit traces). I converted one of these path sets to ProofGrade engrave to see how it looks, and it has changed it to the equivalent filled, closed path.
When I printed it, it did engrave the inside of path (so it’s not just a UI issue).
Curiously, it didn’t fill it where even numbers of these shapes overlapped (there might be some fun to be had there)
Expected: unfilled unclosed paths should not be filled when converted to engrave - the engrave should just rasterise the path itself.
I guess the obvious workaround is: don’t do that. Or, convert to outlines first.