I'm at a loss here.

I have a song that has a trombone as the melody instrument ... or claims it does. It's in the mixer, it's in the notation. There's no instrument embedded in the MIDI events.

BIAB plays a piano.

I've tried re-entering the trombone, I've tried using the GM2 trombone 2, I've tried removing the MIDI instrument and then setting the trombone, I've tried changing styles.

BIAB still plays piano.

I've tried soloing the track, BIAB plays piano including for the metronome (OK).
If I mute the melody track, the melody no longer plays, as I would expect expect.

I've tried muting some or all of the other tracks.
I've tried a variety of other instruments

BIAB plays piano.

Am I being dumb or is there something very wrong? It should not be this hard to set a melody instrument.

Jazz relative beginner, starting at a much older age than was helpful.
Kawai MP6, Korg M50, Ui24R, Saffire Pro 40.
AVL:MXE Linux; Windows 11; Win8.1: Scarletts
BIAB2022 UltraPAK, Reaper, a bunch of stuff.