Thanks, Sandie! As you point out, the inhibitor gene affects the less pigmented areas of the hair, not the tips.
The extra information helps, I think, as I now know that Zeus has the siamese gene. That makes pigment production sensitive to environmental temperature. Maybe there's some kind of "break-through" happening, maybe the result of a modifier. The lighter areas are showing up on a warm part of the body, and this idea might even explain the silver tips that disappeared in the mature coat, just as siamese coats are white at first. Even though the gene should be recessive, there could be some modifier allowing it to have a mild influence.
Good Halloween mystery, thanks to Zeus!