To get things straight: Master selection is always done manually by the user. It does not matter if you use Pulseaudio or not. Thus the whole discussion is moot - unless automatic master selection is implemented, the behaviour you described is expected, and not even a bug.
Phonon is the KDE sound abstraction layer, so KMix should follow Phonon if anything. Thus please follow bug 181652.
To get things straight: Master selection is always done manually by the user. It does not matter if you use Pulseaudio or not. Thus the whole discussion is moot - unless automatic master selection is implemented, the behaviour you described is expected, and not even a bug.
Phonon is the KDE sound abstraction layer, so KMix should follow Phonon if anything. Thus please follow bug 181652.