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I disabled sour notes for the keyboard track. ... And of course I lost the mMaj7 chords

This is "expected". The Fix sour notes / tuning feature (in Song Chords mode) will attempt to "correct" the notes on the track based on the current chord in the song. So that is why you are hearing an AmMaj7 chord at measure 18. What is happening is that 'Natural arrangements' is turned ON (which is the default) and the mMaj7 chord is getting mapped to a m7 chord in this style. The Fix sour notes is taking the Am7 chord and changing it into an AmMaj7 chord by changing the pitch of the G to G#. There are two ways to fix this.

A) Turn off Natural arrangements in the Song settings dialog.

B) Use the Auto-fix (tune) on just the first half of bar 18. I tested this with your song, and it works well. This feature in general works best for specific parts of a track rather than as a setting for the entire track. You could do something like this.
1. Turn OFF the Auto-fix track in Track Settings.
2. Use Track Actions | Copy/Move Track to copy the track to Utility track 1 where you can work on it further and preserve the original BiaB generated track.
3. Open the Audio Edit window, and highlight ONLY the first half of bar 18.
4. Click on the Edit button (in the Audio Edit window toolbar) and click Fix Tuning. Make sure Song Chords is selected and OK. This will add the G# to that bar. Repeat for the other bar of interest.

Andrew
PG Music Inc.