I’d like to ask the good people of this community that, if you do know the solution or figure it out, that you not share the answer here. Feel free to announce your success, but there’s something to be said for discovering the answer completely unaided, even if it takes years.
It’s embarrassingly obvious once you see it though.
Your spoiler is broken (at least on the Lemmy website), it’s missing 2 newlines and a display text. Format is like this:
Turns to this:
Displayed text here
Hidden text here
Without the newlines its just plain text. Note that the “spoiler” behind the opening colons is also needed and not part of the displayed text. I.e. if you want the displayed text to be “spoiler” your first line needs to be
::: spoiler spoiler.