I figured out how to reproduce this issue. It requires minimizing the app rather than simply making it lose focus. It will have a white screen when it is restored.
I think this issue is fixed in the latest source code for Starling on Github. Here's the commit:
With the Feathers SDK, you can manually build the Starling SWC and replace frameworks/libs/starling.swc.
Alternatively, set skipUnchangedFrames to false or set the profile to something other than Context3DProfile.BASELINE_CONSTRAINED.