It turns out that even with many years experience in video editing with Adobe Premiere Pro, I wasn’t aware of the Audio Clip Mixer panel.
I did a lot of work with the Audio Track Mixer, but for individual clips I would set the volume and pan in the Effect Controls panel.
It’s pretty simple to explain the difference between the two panels,
The Audio Track Mixer lets you design the audio mix of a whole audio track, while the Audio Clip Mixer lets you set the mix between individual audio clips in an audio track.
It just so happens, that in Premiere Pro CC 2018, the Audio Clip Mixer is the one that’s open by default, so thinking by mistake it was the Audio Track Mixer, I started using it to set the mix, only to be surprised that the volume I set is changing as I scrub the timeline..
First I thought that maybe automation is on from some reason, and key-frames are being created, but the the key-frame display for the Audio track didn’t show any changes.
I was quite baffled for a couple of minutes, until I realized that the mixing panel I’m working with is actually titled Audio Clip Mixer, and opened the Audio Track Mixer to work as I’m used to.
* One of the reasons it’s confusing is that the Audio Clip Mixer panel displays volume controls per audio track (which makes sense, it just affects the individual clip within the track at the point in the editing timeline)