I know lighter shades can fade, especially in the sun. But not come across this one before. Same client has had white in Ink London and they stayed white, but these are the gel bottle colours. She does do cleaning though so possibly could be bleach? or some cleaning chemicals of some kind.
It could be something your client is doing, many things such as food, self-tan or chemicals can stain the nail. Considering she is a cleaner, there is a good chance it was something she used rather than product breakdown. If however, the same thing happens to multiple clients then I would say something is wrong with the product.
Hi, this happened to one of my clients, but when I went to remove top layer before soaking gel off, it was the top coat that had discoloured and bizzarely only on a couple of the nails not all. Asked about cleaning products used but said hadn't used anything she could think of! Different brand to the one you used, maybe some had more top coat than others or curing time maybe? Mystery to me!