Why does stacking the Whisper style command work in the preview, but not in the post?

Yes. Anything smaller than the first level whisper becomes hard to read. The reason for the difference is that we do the processing on our end of things to make it look as it does in the final version while the previews are just that—previews. They’re done in JavaScript within your browsers and unfortunately it’s not perfectly identical. It’s pretty low priority to fix that bug but perhaps eventually we’ll get to it. For instance making the site faster and preventing downtime kinda takes precedence.