You're hearing a bunch of BS. There's apparently some sort of deal where the plastic, primer, and paint are interacting in a negative way. It should be repainted if it's under warranty. Period. Demand it from Ford and don't give up.
There is only one factory paint job per vehicle.
A repainted bumper will never have the longevity and resistance to damage as the factory paint.
The factory already had years of opportunity and thousands of vehicles to figure how to attain a monochrome instead of two-tone finish.
Mine will retain the factory two-tone patina.