on timeline the mention looks OK. Is there an issue on Yarn?

It's an interesting topic. For example on Bsky it's natural to allow domains https://bsky.social/about/blog/4-28-2023-domain-handle-tutorial

Although TwiXter only allows (letters A-Z, numbers 0-9 and of underscores) https://help.x.com/en/managing-your-account/x-username-rules