I don't know about displaying action/title safe overlays, if shake has them.
If you're rendering using the Animation codec Shake will always save an alpha channel even if you don't want it to or tell it not to. This is my guess as to what's happening.
In FCP, check the alpha channel status of the clip (right-click/ctrl-click on the clip name:item properties:format) - it's probably set to "straight". right click on the word straight and change it to "none"
Or render to a codec that doesn't have an alpha channel.
Without seeing your node tree I don't know why your comp with a solid background has an alpha channel. Even if I did see your project, I don't know if I'd be able to spot problems since I'm pretty new to Shake too.