I have this issue too.
Also running on MacBook Pro Mid-2012, as well as a MacBook Air Early-2015 - Both on macOS 10.15.7 Catalina and rekordbox 6.7.0.
It's as if the program chooses to press command+Q by itself, let's say about 45 minutes in from opening rekordbox.
The issue applies on both Performance and Export mode.
I can't think of a workaround with this, so I have to carefully select 'No' while I'm DJing.
