Cannot read GL_DEPTH_COMPONENT using QOpenGLWidget

[Vagabond]

I am implementing a simple click selection for 3D objects inside a QOpenGLWidget. To do this I need to transform 2D mouse coordinates into 3D worldspace. I had previously implemented the whole thing using QGLWidget. Using QOpenGLWidget, I am not able to read the GL_DEPTH_COMPONENT of a pixel:

float z;
glReadPixels(pixel.x, height - pixel.y, 1, 1, GL_DEPTH_COMPONENT, GL_FLOAT, &z);

'z' is always 0.
To make sure my pixel coordinates were correct I tried receiving GL_RGBA values:

float rgba[4];
glReadPixels((int)p_temp.x(), (int) (viewport[3] - p_temp.y()), 1, 1, GL_RGBA, GL_FLOAT, rgba);

, which returns the correct pixel Colors.
In order for this to work I had to change the domain of the pixel coordinates from local to parent coordinates. This probably results from the fact that the GL_VIEWPORT is set corresponding to the parent widgets size:

int viewport[4];
glGetIntegerv(GL_VIEWPORT, viewport);

for QGLWidget this returns: {0, 0, this->width(), this->height()}
for QOpenGLWidget this returns: {0, 0, this->parent()->width(), this->parent()->height()}

Now I am kind of clueless as to what I might be missing. Any ideas?
Is there maybe an in-built Qt OpenGL convenience function that I could rely on to get the depth component of a pixel?

EDIT: my OpenGL version is 4.5 and I do not get any OpenGL error using glReadPixels with GL_DEPTH_COMPONENT

[Vagabond]

Is there maybe an additional render pass using the QOpenGLWidget, which clears the depth buffer before I read from it? I am certain that I am not clearing it myself before the glReadPixels call. As I mentioned, it worked with QGLWidget.

[Ponzifex]

QOpenGLWidget is works into an FBO. Probably the surface format was set to multisampled therefore the underlying FBO inherited that and reading GL_DEPTH_COMPONENT from a multisampled FBO is not possible. To check this set samlpes to zero in QSurfaceFormat and pass it to QOpenGLWidget.