I'm poking around at it now. I'm guessing it's probably something to do with JavaScript -- which I block by default via NoScript. (That's kind of odd though since I thought it was generating a <details>/<summary> HTML block server side, but maybe it's doing it on the client and I just happened to have JS unblocked when I saw it before?)
Edit: It looks like it is coming from this webpack'd JS file currently which I think is built from this JS source file; there is a handleSpoilers function defined which manipulates details/summary elements. Oddly, there is also PHP code for manipulating details/summary like I thought.
@ernest can chime in on if that's a temporary thing or what, but yeah, it seems to not work for me because I block JavaScript.
NotTheOnlyGamer