DefaultMaterial's specularRoughness default value (50) is not within the range specified in DefaultMaterialSection.qml [0.001, 1]. Either the range is incorrect, or the default value is. Since the range is not documented, only clue for valid range is how it's used in the code. Here's the snippet from defaultMaterialLighting.glslib:
Looks like specularRoughness is used as exponent in pow() function, so 50 seems pretty extreme at first glance.
Also, the stepSize in DefaultMaterialSection.qml is not specified, so it uses step size of 1, which is obviously not correct for the given range. If the range is correct, more reasonable stepSize should be specified.