I think it's just a colourist error. It's not always easy to colour all them frames correctly *is an animation colourist, knows* xP Sometimes a mistake slips through. By the time frames go to be coloured, it's already far past what the animators do, so they are not to blame

There is a similar mistake in the Mufasa's Ghost scene, where Simba's eyes go from yellow to white (or the other way around, I can't remember now) for a single frame.