Export timeline with multiple audio tracks as video with single audio track in DaVinci Resolve 16
The likely culprit is the Audio setting [Output Track 1], which should normally be "Main 1 (Stereo)", which is the default final bus/mix of the audio in the compiled video.

When setting it to "All timeline tracks" -- each audio track (stereo or mono) is rendered as separate tracks (stereo or mono) in the output file.