If you run the PropertyMaps1 demo, the mixture map panel shows that the same material is present as the "most popular" everywhere rather than a spatially varying pattern.
Running the demo with any of the 4.6.x or early releases produces the expected result.
Upon code examination the problem seems to be that the interface to the map access() function was updated in the 4.7.x development cycle to support a time dimension in addition to the XYZ (spatial, spatial, spectral) dimensions. The access() function had a dangerous default parameter setup that resulted in a default time being used for the Z (band dimension) and the Z (band index) always being 0. Hence, when we extracted the spectral values for a pixel (the material weightings), they were all the same because they were all for band (material) 0.
Code was fixed and checked into both the trunk and 4.7.1 release tag.