Audition CS6 - Mixdown keyframes are slightly off

Hi, I'm trying to export a song from Audition. When playing the session it sounds fine, I have a number of volume envelopes to edit out breath sounds on the vocals and the playback sounds great. However, when I either Mixdown Master to a file or bounce it to a track all of my vocal volume envelope keyframes are slightly off and the entire file is about 5 db more quiet.

With the keyframes slightly off, rather than precisely editing out a breath sound, I now hear some of the breath and part of the lyric is cut off, it's like everything is shifted ahead by a few milliseconds.

I don't understand why the mix down doesn't sound exactly like playback on the session. Must be something I'm doing wrong. My workflow looks like this:

Individual instrument and vocal tracks -> Vocal and Instrument Busses 1 (some effects) -> Vocal and instrumental Busses 2 (compression and volume envelopes) -> Master. And like I said before just playing Master within the multirack session sounds perfect, it's just after mixing down or exporting or bouncing that things get off sync.

Thanks in advance!