Michael, you actually didn't do anything wrong, it's just that the Bass track (all tracks except Melody and Soloist) will be overwritten by BiaB when you next Generate the song.

If it's a MIDI track, then I think Freezing the track after your edits will retain the MIDI data. One way that Freezing is performed is by pressing the small asterisk (snowflake) next to the specific track on the mixer. Frozen tracks are shown in blue. Save the song after freezing the track(s).

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