Staff Sammerch mentioned that we explicitly filter out the use of @ mentions when posting to Twitter. We do this to comply with Twitter's anti-spam rules. So it is currently not possible to mention a user using “@”, as flow will remove “@” when post the tweet.
about “Twitter users mentions” could be your reference: https://powerusers.microsoft.com/t5/General-Flow-Discussion/Twitter-users-mentions/m-p/7173
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