Let me see if I can explain better.
I setup the softsynth as a Midi Source, Track Folder, First Synth Audio Output. Basically setting it up as a non simple instrument track.
I record three notes - C, then D, and F. Everything plays back fine. I then change the D to an E in the PRV. When I then play back, I hear C, then D AND E (at the same time), then F.
If I set up the softsynth as a simple instrument track, and repeat the same test, I only hear C, then E, then F.