To test whether the problem is due to your SVG images or something else, I created a Greenwade set on the Chess page, and it had the same problem. However, when I started to reply to your comment, it displayed the pieces correctly. And when I went to the preview of my comment, it had the same problem as before. So, maybe something about the surrounding HTML and something about your SVG images are both at fault.
To test whether the problem is due to your SVG images or something else, I created a Greenwade set on the Chess page, and it had the same problem. However, when I started to reply to your comment, it displayed the pieces correctly. And when I went to the preview of my comment, it had the same problem as before. So, maybe something about the surrounding HTML and something about your SVG images are both at fault.