MIDI export shenanigans! Need help troubleshooting

I have a score that I am trying to export for use as a midi file, but the parts don't seem to get along with each other. If I include the bass part when I export the main score, the trumpet part turns into a midi electric bass. It seems that any time I put the bass part and the trumpet part on the same MIDI file, that is when the trumpet turns into an electric bass. But when I remove the bass part and then export, all the instruments behave as they should. What's going on here?