I have the Roland TD-10 exp, and I am using this circuit to split port 12 (Aux2) for my splash and china. I get the same triggering issue as you, however I also had the china cut short as if it were choked, so I placed a diode between VR1 (the 100k trim pot) and D1(at the loop) so that it keeps the flow going and doesn't bleed back. That stopped the choking misfire sound, but instead, I simply don't get any trigger when I would have gotten that. It cleaned it up, but I have to figure out how to get the voltage right to always route it correctly.
Any thoughts on this?