With some fonts, when you give them a stroke effect, it only looks good in the design tool. When you check the preview, however, it’s no longer clean. As if you need to click “round join” somewhere to round off the corners.
How does it print? Will it look like in the design tool or as in the preview? And if it’s in the preview, can you change the stroke corner settings to “round join” instead of “miter join?” At least, that takes care of the issue in Illustrator, so I suspect it would be similar in this tool.
Not only does it show on the previews, but also on the downloads, so even if you make mockups/covers it is there.