@TheFinite
Not a mod or dev, just posing some questions I think could be relevant:
Does spoiler text show in other threads, too?
Are you using any browser plugins or custom themes, eg. some kind of “night mode everywhere”, “reader mode” or similar? Or, accessibility settings to enhance text contrast? Because, the spoiler text hides itself by having the text the same color as the background, and something that changes colors would mess that up.
Could you post a screenshot of what it looks like for you?