Figure 51. Internals of a zero-cross detection circuit based on the G3MB-202P. It's not so simple really! Schematic based on that in EDN article and referenced in my answer to Using AC current to trigger Triac
If \$(V_{L1−L2}\$) is low (above but close to zero) and Q1 is turned on by photo-action from D1 then SCR1 will be triggered. This in turn will pass enough current through R6 to bring TRI1 gate voltage high enough to trigger.
When voltage exceeds a certain level Q2 will be biased on. The collector voltage will fall and there won’t be enough to turn on SCR1 even if Q1 subsequently turns on. The effect is that TRI1 can’t turn on unless it is triggered close to the zero cross.
The problem you might experience with operating on low voltage is that a certain minimum voltage is required on L1-L2 to make the circuit operate. At full mains voltage that's happens very close to the zero-cross. As the AC voltage decreases that voltage is reached later in the cycle and zero-cross action becomes less and less true!
Figure 2. Possible resulting waveform for zero-cross opto-isolator switching a low-voltage supply. Image source: mine.
For additional material see my article here.