Never use MIDI Mapper unless there is no other choice.
That alone could cause your issue. PGMusic has said a couple times that simply using the MS GS Wavetable synth as the default instead (even if using a soft synth in the long run) has solved some issues like this.

Herb's solution is preferred, but thought I'd mention the above in case it helps you or others.

I do not work here, but the benefits are still awesome
Make your sound your own!