An Integrator Reset Circuit not working properly
The gate source capacitance of the J113 JFET is a few pF (5pF max) so you'd expect the gate voltage delta to be reflected in the output voltage by about the ratio with your 66pF cap.
Which is what you are seeing.
If you don't like that, you can design or buy an analog switch with less charge injection. A good one might have typical injection in the 0.5pC range. Yours is almost 50x worse.