So I'm using the clock transition in FCPX, by adding a storyline to the clip I'm applying it to.
I then adjust the settings to the inspector to my requirements, changing the centre points and adding an edge feather.
I skim through and it's the result I want. FCPX then renders in the background, and I look at it again and the edge feathering has gone.
My take is that FCPX as of now has some issues with render files. I'm seeing this kind of artefacts too, mostly related to transparency when rendered. Re-rendering sometimes fixes, and sometimes not. Hopefully ironed out sooner than later.
Technical Director, Media Technology, University of Gothenburg, Sweden