Hi Michael
no body is taking away your protection, all I'm saying is that your zener diode only works as a normal diode because the voltage across it never reachs the zener voltage due to the ON voltage of the photo diode in the optocoupler which is about 1.8v max.
So a normal diode works better than a zener diode in this case and cheaper too.
Have you connected a scope across the optocouplers photo diode and found noise level exceeding 1.8 VDC?
Mahmood