When a user opens a shared Muse link, DNS resolvers observe and log the full hostname including the title and random ID into public, searchable passive DNS records.

From the articleMeta Muse exposes users' personal and business secrets when they share conversationsAlso in Meta suspends Muse chats sharing feature after it left private details exposed - Engadget, Meta Suspends Muse Chat Sharing After Researchers Find Links in Public DNS