The leaves on this one (and thanks for asking this question, I was unaware of this new variety) turn yellow in autumn before they fall. This plant also likes moist soil, doesn't like to dry out, and the weather has been very cool and autumnal - it may be the plant has decided it is, in fact, autumn (which it will be in a couple of weeks anyway) and has started to change colour.
My only query is the green dots and squiggles still left on some of the yellow leaves - have you checked the back of the leaves and all the stems for signs of infestation or disease?

my normal one is just starting to turn yellow. it is a wonderful plant for autumn colour.