My 10 month old ISA browns have (and have always had) white earlobes. They still lay brown eggs and are perfectly healthy. The white isn't a disease. I have always assumed it was just reflective of what breeds were crossed to produce a sex linked hybrid (ISA brown).
Of course if anyone has other info, I would love to hear it too!

@purrcoaxx this is one of my girls, do yours look similar?