It seems there is a bug with the new Flow preview connector for Teams that allows the Flow Bot to post messages to users and channels. It will not post a clickable link. All links are converted to plain text. It doesn't matter if you use the <a href=""></a> style link or the [Link name](http://blahblah) style markup, everything is shown as plain text. This includes links that you can insert in a flow via the Dynamic Content, such as a link to a Power BI tile. That gets converted to plain text.