Thanks for looking into this. It makes sense now. I discovered this when trying to use Vienna Jazz drums using the default GM percussion map. But Jazz drums, while GM compatible, has a much narrower range of notes mapped to samples. So I was getting no playback on some notes due to the conflict and also when selecting notes. It was triggering notes out of range.
Anyway, I made a duplicate percussion map and removed notes below Midi note 35 and above Midi note 60 and all is well.