The preview-hidden-comments feature isn't working probably because GitHub started hiding some reasons why a comment has been hidden:

I'll add and hide comments below as a live example.

Here's the originally-allowed types for reference:

https://github.com/sindresorhus/refined-github/blob/c6c727ca571c91d1ff426e218ccadb075cdb2af6/source/features/preview-hidden-comments.tsx#L9

Originally reported in #3621 (comment)