NOT confirmed. Works fine here. [EDIT: Wrong, see newer post].
I also tried it for different mixer settings which could impose focusing: Always on top, Full screen mode, or none of these. It works fine any way (single screen).
The only way I could not use Solo/Mute shortcuts, with Channel Edit opened, was when I happened to click the Mixer Visibility tab outside the channels, so no channel was selected any longer. But I suppose that channel deselection then is intended behaviour.
Logically it should keep the channel selected if the Channel Edit window is opened, even if clicking in the "nothing" area of a Visibility tab.
Cubase 7.5 64-bit, Windows 7 64-bit