This is usually caused by the opto-coupler’s output ringing because it is driving too high of an impedance. We have found that some opto-couplers are more sensitive than others in this way. The fix is reducing the value of the pull up resistor on its output (R18). If you replace R18 with something less than 1000 ohms, it should help….Alternatively, if you have another resistor (anything between 300 ohms to 2500 ohms) you could put it in parallel with R18 and the parallel combination will be under 1000 ohms.