Two possible reasons that come to mind:
- Class-X capacitors achieve their safety by self-healing, ie. if a spot in the dielectric film breaks down, the sputtered metalization on each side is thin enough to burn back from that spot to clear the short. To do this, there needs to be sufficient current available to achieve the burning. In across-the-mains applications, this current is obviously available but not in high impedance series applications, so the short could pass a lot of current into your opto.
- Class-X capacitors are deliberately made with thin metalization so that they will self-heal easily. This limits their series current capability and causes them to loose capacitance over time. This is a common failure mode in capacitor dropper PSU applications.
I'm not sure which it is for this particular manufacturer, but I think it is one of the above.