I would check the following for evidence that might point to the cause of the failures:
One problem with hot water recirculating pumps is that they are sometimes oversized and produce very high velocities in the pipes for very long durations. What is the flow/head curve for the pump, and what is the flow rate at the head that exists? What is the size of the pipe? What velocity in the pipe? How many hours per year do the pumps run?
Are there any signs of erosion at the locations of the failures when the failed pipe is sectioned and the inside surfaces inspected? Is there any sign that the protective films are damaged by erosion at local areas?
Are there any materiels such as sediment from the water heater that could be circulating and causing erosion?
How far from the pump are the failures occurring? On the inlet or outlet side of the pump?
Are there any signs of corrosion at other locations near the pinholes.
What type of pipe? Copper L or M?
What are the materials in the pump and pipe, and in the water heater. Are there any dissimilar materials that could contribute to electrolytic corrosion?
What is the pH of the water? What temperature?
Is there any treatment that could have altered the chemistry of the water?
Is there anything about the location of the failures, such as at elbows or reducers or bends, that might give a hint as the cause of failure.
Are the failures associated with support points where vibration might be causing damage?