How do you have ModernMix configured on the Advanced page in the configuration?
There is a setting to indicate what happens when you close fullscreen applications.
There is also a setting in Start8 under Desktop->Advanced button which if set will make it return to the desktop on Modern app close.
Thank you for your response.
Start8 has 'Switch to desktop after closing a modern UI application' selected
and Modern Mix has 'Only when closing full screen application launched from the Window8 Menu' selected
Thanks again.