This looks like a bug in the Glowforge software to me, but:
I started with the MilSpec33558 font, used Inkscape to write some text and “Object to Path”, and I got this:
Then I uploaded this to the GFUI where it looked fine:
But then when I engraved I got this:
The center of the A and the section of the A’s right arm that is next to the center have all been engraved out to about half the depth of everything else. This was on proofgrade maple.
Interestingly, after you click “print”, the preview shows a filled-in “A”.
This is an SVG file which reproduces the problem: