Tested, and you're right, there is a problem. I tried to edit (with and without editing the text/letter) and checked the previews, I got random results with working and not working sizes. I tried then to change the font to a normal one, and in this case, all previews were correct but the preview thumbnails in the size details partially wrong. No problems so far with the tests of my own designs (images).
That being said, you had a pretty uncommon approach with the symbol font. A "risky" one, to be honest. Be aware that the font sizes change when you change the product size, and this may cause trouble. If you look at your design, there are white (unprinted) horizontal stripes between every text line, both in the previews AND within the designer, where it matters. You could adjust the line height, but this is not precise enough to get a clean result. I'm 100% certain this will print that way and you could risk a return. I personally would redesign it using either tiled seamless vectors/images or a full-size background.
