If you start an X / Twitter post with a mention, it will be considered a reply. This is based on how X handles @mentions at the start of posts.
In fact, replies appear in timelines only for the tagged account and any mutual followers, and won't appear on your profile unless you visit the "Replies" tab.
If you'd like your post to appear on your profile as well as people's timelines, you’ll just need to add at least one character or symbol in front of the mention.
How to make sure your post appears on your profile
If you wanted to post the following on your profile:
@typefully is so great!
All you need to do is add anything but an @ symbol in front. Like this:
.@typefully is so great!
As you can see, simply adding a “.“ in front of the @mention will make it so that your post appears on your profile instead of in your replies. In case you’re having trouble with this or anything else, please reach out to support or check one of our other help articles.