Ah wait, wasn't the advantage of the CoronaMix map that it works correctly with bump? I remember something but not sure what exactly :D
You might be really seeing two different special cases here - first being that bump together with CC generally need CoronaBumpConverter to work correctly and that CoronaMix map works with bump - which I'm not sure about in detail.
So, bump in general needs special treatment... would be interesting to sort out what happens where that's why a color channel should be tested, too.