This annoying bug persists from C5 to C6.
If you have “tapepachine style” monitoring turned on when you do an Export --> Audio Mixdown, the track selected is muted.
Tapemachine style monitoring is supposed to monitor your audio input (usually your mic) when you are stopped or recording, but play the track when you press Play.
So naturally, an Audio Mixdown is like hitting Play.
But if you have an audio track selected and choose Audio Mixdown, the monitor button doesn’t turn off as it should. Instead, after you wait for your mixdown and open it in WaveLab, you discover that the audio track that happened to be selected at the time of mixdown is muted.
It’s not deadly… but it sure is unprofessional and annoying.