IF you are using multiple monitors(?), Microsoft say this
" The issue is that windows views multiple monitors as one desktop.
When you interact with a Metro App,. It is on the same desktop, although they appear on different screens.
This can be replicated by opening a metro app on a split screen with a desktop on the other"
This sounds a little like a rather poor excuse. I would prefer to call it a bug.
I cannot personally test it. I do not use any Metro apps and, which may have an influence, I have modernmix on other computers to which I have access, which may help in eradicating the bug.
If you are not referring to multiple monitor problems, then Microsoft's only (weak) suggestion is that you update your graphics drivers and experiment with other resolutions.