Yeah, I see that too. I bet that’s the problem. I just took a ruler to it and, sure enough, 39 lines per inch. I set it up to 340 and switched back to the Proofgrade setting and it defaulted back to 39 (and the preview showed it.) Trying an engrave now in manual mode at 340LPI.
When you upload an image to discourse, it selects a default x-y size of the image. You can change them to make the image preview larger. In the example below you will see the 378x378, increasing those numbers of the link code increases the image preview.