I've noted this too with my pair of JL Langton in New Gold Museum. Although I'm not too concerned - perhaps I should be? Photo taken today:
These have been in the rotation for three years now. I've probably exacerbated the problem by using Neutral Saphir Medaille D'Or Wax and Cream, which leaves behind some white residue in the creases. An application of Renovateur gets rid of the residue.