https://github.com/discourse/discourse/commit/61bb0df3f638963e331af4c4722216695ad6b295
I prefixed the commit with FEATURE because I couldn’t find anything in the codebase that suggested this worked before so the only way to “fix” this was to add support for it which means it’s a new feature 
I also noticed that toggling the pin from topics lists wasn’t working here on Meta due to a theme component so I fixed that as well:
https://github.com/discourse/discourse-topic-thumbnails/pull/13