When you export with an alpha, make sure you set the alpha channel type correctly in FCP. Ctrl-click the clip in FCP and choose ‘Item Properties’ Set the alpha channel type to whatever is appropriate for your clip.
I would recommend exporting anything out of AE with a “straight alpha” since some other NLE, such as Avid, don’t easily support premultiplied alpha channels.
Michael Lange
Producer/Editor
https://www.langetv.com/blog