PhongAlphaMaterial not working within Scene3D

Changing PhongMaterial to PhongAlphaMaterial in qt3d examples that use Scene3D item ("scene3d", "controls") leads to disappeared entities ("scene3d") or slightly visible contour ("controls" example):

In examples without Scene3D (basicshapes-cpp, simple-cpp, simple-qml, materials-cpp) PhongAlphaMaterial instead PhongMaterial works as it should (ambient color, alpha parameter - all ok).

I am writing my own application using qml Scene3D and everything inside it on cpp. In qt 5.6 PhongAlpha worked, but there was another problem (http://lists.qt-project.org/pipermail/interest/2016-May/022352.html) as I understand recently it's exactly due to PhongAlpha, with Phong everything was fine.
Now I move my app to qt 5.7, it works with Phong, but PhongAlpha is look like white Phong - it ignores ambient and alpha.

Update:
I just understand that such behavior (with white Phong) obviously due to the fact that my Scene3D qml item is inside Loader.
I can get same behavior from "scene3d-loader" example (for now it is in manual test): just change PhongMaterial to PhongAlphaMaterial in AnimatedEntity.qml and set clearColor to "light blue" for RenderSettings (to clearly see that entities become white without transparency):

And the worst in all this situations is that PhongAlpha with alpha parameter 1.0 works in different way that Phong. I can't understand it at all, cause I see that phong.frag differ from phongalpha.frag only constant alpha = 1.0 parameter

gl_FragColor = vec4( ka + kd * diffuseColor + ks * specularColor, 1.0 )
vs
gl_FragColor = vec4( ka + kd * diffuseColor + ks * specularColor, alpha)