Replying to a topic or quoting some topic content (by marking the content to quote) did insert the @username of the topic poster in all previous versions. Since the update to 1.9.9. this nice feature is gone.
I feel this is a bug or is this intended for a worse user experience 😉 🤔
This is not a bug. I think you've disabled the "Show Member Mention Nicknames" option in Dashboard >Forums > Settings > Features tab. Try to enable it, we've connected the @nickname inserting feature with this option in 1.9.9 version: