I don't think it's anything to concern yourself about....I *think* it's just an area that doesn't have the 'glaucus' (or however you spell it !
) coating.....that "waxy" surface. if you 'wet' the leaves, I don't think you will see the water 'run off " that area...in other words, that part of the leaf WILL get wet, instead of shedding the water. I see it , sometimes, and I *think* it's caused by the same factors that cause 'pineappling".....cold snaps, or "physical restraint" keeping the new leaves from emerging from the fans easily.