I've built a few of these over the years.... Vero, etched, and fabbed PCBs... All of them have exhibited this same problem.
It's worth noting the schematic for the original pedal as found on the patent application doesn't match the layouts/schematics floating around the internet...
There are several differences.... the Zener diode D5 in the feedback path, the absence of C2 and C18, the slightly different orientation R7...
I would be very interested in adding a capacitor in the location of C2 to your pedal and see if it helps....
Schematic from patent application