It appears that using MathJax color in a hidden text block "pokes through" the hidden environment. I don't believe this is desired behavior, so I'm reporting as a bug.
very hidden. $\color{red}{\text{Not so hidden...}}$ Hidden again.
It appears that using MathJax color in a hidden text block "pokes through" the hidden environment. I don't believe this is desired behavior, so I'm reporting as a bug.
very hidden. $\color{red}{\text{Not so hidden...}}$ Hidden again.
I don't know why you're using colored text in spoiler blocks, but the obvious solution here is to just not use colored text in spoiler blocks.
Spoilers are fairly reader-hostile by-design, so if something about them strikes you as odd perhaps you could just skip using them entirely.