Some of our compositing elements have transparent backgrounds, so that you can use them right away with normal blend mode. However, Final Cut Pro does not fully support creating proxy/optimized media for video clips with alpha channel, and in effect transparency is lost in the optimized media.

To resolve the issue please remove generated optimized media (select the clip in the library and run the File > Delete Generated Clip Files command) or import clips with automatic optimized media creation disabled and make sure that in the View menu (to the top-right of the viewport) Media Playback is set to Proxy Preferred or Optimized/Original (and not to Proxy Only).