Did you or @MacMannes ever resolve this? I breadboarded the Schmitz “Fastest Envelope in the West” today and it similarly doesn’t return to 0 V, I tried adding the diode and 100k resistor and that made it get back to 0 V or at least a lot closer. Downside is it reduces the amplitude of the envelope a little including the sustain level, but maybe that’s acceptable.
Blue = before diode, yellow = after.
I’m surprised you didn’t see better results, but maybe something else is going on.
I’ve just now found Kassutronics’ solution to the problem (with the Schmitz ADSR), it’s interesting:
This design takes it further, getting rid of the remaining ~20 mV using a MOSFET switch, but that’s probably more trouble than I’d find it’s worth.