Preventing op-amp output fluctuation at power-off event
After power off behavior is not guaranteed, you can't predict what will happen. The thing that is most concerning is the jump happens when the rail voltage is lowest.

A few things that might solve the problem:


Use a different op amp than the LM324 which is old
Use an op amp with an enable/disable pin
Clamp the output on power off with a mosfet pulldown (that won't burn out the output of the op amp).