Chuck I agree..damage from cavitation was my thought from the get go. I have repaired many issues caused by electrolysis but this damage is much too specific and quick to fit that bill on its own.
Also don’t disagree with you and many of the other posters that there may still be some dissimilar metal corrosion at play..
I’ll grab a pic but the wax mixing valve is 2 1/2” and while it’s possible to go back far enough to grab 4” copper, I’m not sure that’s going to accomplish anything in regards to a larger suction line
Are both pumps piped in the same? Does the one causing the problems have a 90 or other fitting closer to vanes and volute? If electrolysis was doing that, people would be getting "shocked'" from touching floating grounds with that much circulating currents. Let's face it, almost every electric pump circulating system will have "stray" currents circulating to lesser impedance to earth grounds. You have a turbulence or suction staving problem.
