Is your instance Lemmy or some other fedicerse platform? And if it's Lemmy, what client are you using? Because their syntax displays correctly for me on Jerboa and lemmy-ui, and I'm fairly sure it'll work on any well-behaved Lemmy client.
Ok so it looks like Piefed is the name of the platform you're on, a different threadiverse platform from Lemmy or Kbin. Unfortunately some of the more advanced syntax like spoilers are the first things to become incompatible when you cross over between different platforms.
This should work on Lemmy:
Test
Test spoiler text
This should work on Kbin, I believe:
Test spoiler text
!This is the syntax that worked on Reddit.!< (and tbh I quite like that syntax because it enables inline spoilers, instead of only paragraphs)
From what I can tell, Piefed has similar behaviour to Lemmy. Unfortunately emotional_series is on Kbin which means their spoiler syntax is breaking for you, and for me. Their above comment is particularly bad for me because they didn't even include a paragraph break before starting the ::: spoiler. And it breaks even worse for you because Piefed seems to default to treating the text that Kbin intends to be hidden as the revealed part of the spoiler if they do have the paragraph break. And it also ignored code blocks in order to spoil code.
This is what you should be seeing in the lower comment:
This is incredibly helpful, thank you for taking the time to help! I honestly thought Piefed was Lemmy, I'm still new to the fediverse. I think I should try using an app
Yeah the great thing about the fediverse is how interoperative it is. There are differing degrees of compatibility, but to some extent we can talk to each other even between Mastodon (ostensibly a Twitter-like platform) and Lemmy (Reddit-like). You could never imagine Tweeting to reply to a Reddit post.
The fact these platforms all play differently is so annoying. First with linking other magazines/communities, now spoilers apparently. Something that looks nice and works on one platform is unreadable/spoils/nonfunctional on another. For what it is worth, all attempted spoilers in your comment that I am replying to now work for me on Kbin. Except the Reddit-style spoilers, which I believe work nowhere on the Fediverse.
The reason I included the Reddit one is because I think some Lemmy mobile clients that used to be Reddit clients might still support them. Because really rendering is a matter for the client more than the platform.