Attached is one of our card SVGs. It behaves in a way I'm left to think is a bug unless there's some normal reason for this behavior that I'm overlooking.
The card has a few layers:
Text - The main layer where most everything on the card ended up so it's not so much text as really what's become "the top layer".
Poison - A special layer unrelated to this card and blank here.
Background - A critical layer to this issue. It generally contains just an image of a moon and I applied an Opacity of 80% to it when later we felt the moon was too dark in printing and this became an easy way to adjust it over various test print runs. It also provides the white background that is not there when this layer is off that, without it, results in a see-through card as you see below. Necessary for printing onto white card stock, but not desired for non-printing previews.
Border - A layer that is 99% of the time turned off that contains a white-filled, rounded corner rectangle with a black stroke. We turn this on to give the image the look of the shape of a card, but keep it off for printing as the cutting process creates that "border" and so the black stroke border is not desired. However, this is also a major part of the trouble. This layer is BELOW the Background layer, but if we turn it on it results in the moon in the layer above it to vanish entirely. I can't explain this.
Base Template - This layer is off almost always after the initial setup and is just the image of the printing guidelines for spacing and edge limits, etc.
We've just recently worked around this anomaly by setting the PNG of the moon to a lighter shade that matches the 80% Opacity look and then returning the Background layer to 100% Opacity which then results in the Border layer working as anticipated. I can turn it on, see the black border around the card, with the moon, and then export a PNG to use this anywhere it's needed. I still want to know why this is happening though for future us as this approach was so darned useful.