Oh, maybe I should clarify! This is not a repeat problem with the same motor. I was talking about cap failures in many different electric motors, over the last 45 years or so. It appears to me the motor mfgr’s often skimp on the caps, relative to the rest of the motor.
You are of course 100% correct about the other checks — all looks great.
My conspiracy theory about this is that the mfrs use substandard caps so when they give it up, likely from heat over long periods of time, most people will have to buy a new motor/alternator/etc.
Best answer is switch over to three phase.;-)