It hasn't since the beginning. This is because it's still using and expecting Reddit's spoiler markdown. Should be fixed next release. See this earlier post from August:
Eternity's specific issues around spoilers aside, I think other apps / platforms might struggle with that post because the spoilers aren't technically formatted correctly - the ::: that ends a spoiler should be on a new line.
lemmy-ui might've handled them as the user expected, but anything using a different Markdown library might not (including Lemmy's own backend when it comes to the HTML content field for federation)
They were formatted correctly. This screenshot here is of the rendered text, not the source. The issue is that with markdown, you need two newlines to create a new paragraph. If you only use one newline, the subsequent line just gets placed at the end of the previous line. But the Lemmy spoiler syntax does not work that way; you can put the ::: delimiters directly above and below the spoiler text without the need for any extra lines. Because Eternity does not recognize the spoiler syntax here, it is placing all three lines of each spoiler into one line just like it would do to any arbitrary text.
Ah, I see now, thanks. I didn't know where the original post was, or I would've confirmed. I suppose I could have figured out it was for Forza Horizon in retrospect.
You can get a new line (as opposed to a paragraph) with Lemmy using 'space space enter' but I didn't realise that spoilers don't even need that.
Lemmy follows CommonMark, which is similar but not exactly the same as what was used in The Other Place. I do wish Lemmy supported some form of in-line spoilers.
Markdown itself is very old and afaik does not actually support spoiler tags (I don't think they were even a thing at the time). So every implementation of spoiler tags is just their own cooked up mess.