We were unable to reproduce this here, after the following music track starts, the voice track bed fades out properly for all cases: when the bed is assigned for voice tracks, or when it's assigned for a file type with the "treat as a voice track" option.This is the typical scenario - voice track plays as the song fades out (triggered by the song's outro marker) , and an accompanying music bed begins playing in the background after the song has faded out.
The fade out is performed according to Settings / Playback / Voice Tracking / Fade out option.