I’ve slipped this over to Contribute > UX as I think it’s working as intended. I think a change to the setting description could make this clearer though. It’s currently:
topic excerpt maxlength - Maximum length of a topic excerpt/summary, generated from the first post in a topic.
We do have this on a couple of similar ones - ‘If you change this, you must rebake all posts with: “rake posts:rebake”’, though I don’t know if we could improve on that while we’re at it?