I don't think it's SVG. It's likely the way the heart creates its gradient with a referenced image. The thumbnail webkit instances are rendered, then snapshot, encountering sandbox and timeout limitations. Inspect the SVG code to shed light on the matter.
I think the problem… which is why I say it's extremely minor… is that it's added via HTML in an element. Also, it's an SVG image. That's not a common way to work with Hype.
I forgot the exact reason why I did it that way.
It's also a minor issue because not game breaking, because players don't usually see the Hype thumbnails for scenes.
I'm just reporting the issue, because I think Tumult prefers that. The real blocker is all the Web App stuff.
Yes, thank you for reporting the issue! You're welcome to send the .hype document (I didn't see it on the page), but the general issue of SVGs not displaying in the thumbnail view is a known bug. It has to do with webkit rendering offscreen in some, but not all, circumstances.
(I don't think the WebKit team will ever fix this specific bug since they have a more general solution; unfortunately that general solution has other tradeoffs that don't work so well for Hype's thumbnail generation...)