QSkyboxEntity breaks rendering on 5.15

In Qt 5.15.2 (LTS) and below using QSkyboxEntity (C++) in a scene breaks the rendering:
for some objects it looks like face normals are inverted which results in showing the backsides of mesh faces that should not be displayed, but not the front side (see attached video).

It seems to work in 6.5, though.

I attached an example project that works in 5.15 and 6.5 and shows the bug in 5.15 but not in 6.5.