Embedded Linux QT 4.5.3 [Repaint artefacts under transparent widget]

I have three widgets as follows:

1) opaque widget
2) widget on top of the (1) contains semi-transparent Pixmap
3) widget on top of the (2) completely transparent (Alpha is 0)

2 and 3 children of 1.

Everything is fine until I start moving widget (2). When it's moved updated region appears as black filled rectangle instead of semi-transparent Pixmap of widget (2).
I found that if Alpha for widget (3) is in [8..255] range everything works fine otherwise [0..7] I get these black filled rectangles.
Furthermore there is no such problem on Windows QT.

I went through Qt Embedded Linux source code responsible for painting and it appears that embedded layer gets image with artefacts. That means that something happens during widgets composition.

Spent some time trying to find where the composition take place in the QT source code and what exactly happens there but so far without success.

Of course it's possible to set transparency higher than 7 for all widgets but I would like to understand the cause of this problem.

I would appreciate If someone could shed a light on this problem.

PS. As I mentioned I got this problem on QT 4.5.3 for Embedded Linux on ARM platform

Thanks