What I suggest, and which mirrors practice on the (possibly quite non-representative) slice of discussion fora I have participated in over the years is:
Clearly label sensitive information. In other fora, this often involves linking your username to some real world information-- I'm thinking of old timey mailing lists, or fora where people just sign with ...
Re order the tags:
>! <sub>subscript spoiler</sub> | normal spoiler
The line needs to start with >!. You can put subscript inside the spoiler, but you can't wrap the whole spoiler in subscript with <sub>>! spoiler</sub> like you were doing.
Talk to your players
Tell them that you are asking these sorts of questions and ask them to avoid looking at those while enjoying the site.
Also, follow @Novak’s advice in case the players trip over it without looking for it. As for point 1 of that answer, just tell them your username.
Ultimately, if they want to spoil their dinner, you’re not their mother.
The linebreak before "If" breaks the spoiler markdown.
Curse of Strahd spoilers ahead.
Fixed (by deleting the linebreak), it looks like:
Alternatively, we can fix it like this:
With a double space at the end of the first two line in the spoiler block. This is the solution I went with, as this matches the rendering in the adventure's text.
After seeing the related Q&A on Main Meta (thanks to Rubiksmoose for finding it): Line after blockquote in spoiler not a spoiler?
I have solved the problem I was having by adding a <p> tag after the nested quote.
Specifically, what I had before (what I used in this meta question) is this:
>!This is a spoiler quote and is hidden.<blockquote&...
The linebreak is indeed what broke the spoiler format.
If you wish to still format these with a new-line within, simply include two spaces after the first line and readd the >! to the following line.
>!Spoilers followed by a double-space
>!Following line after line-break