Paul,
I have to admit that my statement should not be taken too general. Water quality, e.g., might affect this as well as other factors (age of chemistry, spreading to other process stages, etc.). But I can easily reproduce this with, e.g. Provia 100F and E200. The magenta (or magenta-red) cast on the Ektachrome is clearly visible on the light table, but can be filtered on reversal or digital prints (no crossovers). I've not tested all possible combinations of Fuji and Kodak (or other brand) slide film, but made this expericence a while back and avoid processing different brands in Tetenal E-6 3-bath since then.