I found it! The problem was a little different than the one you suggested, but you got me looking in the right direction.

I'm still exploring this tool (there's a lot of stuff here!) and I had earlier created a "sing-along" part, in which I had selected the tenor part and had given it a synth-bass patch. Later, I went back to my regular conductor's score and I had forgotten about my sing-along experiment.

When I played back the music in my conductor's view, there was that obnoxious synth-bass in the tenor part and I didn't know where it was coming from.

I really love this notation tool. I've used quite a few before, but this one is more powerful and flexible than I've had before, without being too complex. And thanks for your rapid response. Prestissimo, I guess I should call it.