It looks like Sterling K. Brown singing in the audition was just done to make sure he could if and when that was needed.
It’s far from uncommon for songs to be added and cut during the production of an animated film.
Several of the unused songs in Frozen II were released on the soundtrack, though this particular song was not.
What’s interesting from watching Into the Unknown is seeing that “See the Sky” was actually a somewhat late addition to the film, before being cut out.