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.
details tag with proceeding newline
This text is hidden until you click the text.
Here's an embedded
details tag embedded
This text shows up all the time in the page, but the preview makes it looks like it works.
Mystery fucking solved on why the spoiler text sometimes worked and sometimes didn't.