How is this optoisolator being sensed by an input?
This circuit uses a power resistor to drop up 360Vpk or 5mA into Full Wave Diode Bridge then LED. The output current thus saturates the effective Darlington inverter so that only zero crossings are pull-up except when the input current drops below the threshold which may be in the 1mA range.  Thus it is possible to generate a 1% zero-crossing positive pulse at 2x grid frequencies.
simple simulation.

I omitted the MOV on the output stage.