The auto-mixing option to the material map (mixing="true") is intended to do a (quantized) mixing of the two closest materials in the map based on the relative digital count spacing if a given DC is not included in the LUT. This works for two materials, but multiple materials are ignored.
This bug can be demonstrated with the MaterialMap1 demo by swapping the two middle entries on the LUT -- the gradient should change to linearly decrease from the left, increase from the right and meet up in the middle -- it does not.
This was tracked to a typo in the mechanism used to track the DC distance to the nearest defined material. Instead of using the index tracking the next material, the previous index was being used instead.