The ABS sensers are magnetic, not optical.
The problem could be caused by the axle you have not replaced yet.
The other three axles have new reluctors on them and the sensors will be getting good signals because the reluctors are clean.
The remaining reluctor is dirty and rusted and my not be providing a strong signal as the other three are.
This can be verified by connecting a scan tool to the car and looking at the data stream from all 4 wheels while driving.
