I'm not sure what revision(s) of the answer you're talking about, so I may be off the mark. You cannot have multiple consecutive paragraphs in spoiler markup. (You can have line breaks inside a spoiler paragraph, however.)
This is by design: if you have a large, multi-paragraph block of spoiler text, you're hiding too much. You should only hide actual spoilers, not your whole answer. I can't find an officially status-declined report on that, only a statement that “it's not fully classified a bug as much” and “That's somewhere between a bug and a feature request then. It's kind of by design, but more of the "because that's how it works" variety.” in one of the several times similar issues have been reported.
There is a workaround, which is to use
<p> tags for a paragraph break. This works by happenstance, not by design; I wouldn't recommend relying on it.