Unfortunately, switching cubemaps during a blend fraction of 0.99999 still results in a pop, since it’s not actually using 99.999% of the target cubemap. At least not in the reflections.
Even at 0.99999, the influence of the source cubemap is still very large, so switching it out results in a noticeable difference.
But thanks for looking into this either way. If it has been around since 4.8, I suppose the chances for a fix are rather slim. I’ll try to work around this issue by using more cubemaps and few reflective materials, so the pop isn’t quite as pronounced.