In Discourse 1.8.0.beta13, the [details=Spoiler]This should be hidden[details] logic doesn’t work if it’s embedded in a paragraph. It does work if it’s separated by a newline. The problem appears to be in the generated html, as it’s reproducible in Chrome, Firefox, and Safari.
details tag with proceeding newline
This text is hidden until you click the text.
Here’s an embedded [details=details tag embedded]This text shows up all the time in the page, but the preview makes it looks like it works.[/details] paragraph.