This 'old' code correctly sets the bit depth:

QGLFormat gfrmt;
gfrmt.setRedBufferSize(6);
gfrmt.setBlueBufferSize(6);
gfrmt.setGreenBufferSize(6);
QGLContext *glc = new QGLContext(gfrmt);
glc->create();
qDebug() << "QGLContext red buffer size:" << glc->format().redBufferSize();

This 'new' code always uses 8-bit:
QSurfaceFormat ogfrmt;
ogfrmt.setRedBufferSize(6);
ogfrmt.setGreenBufferSize(6);
ogfrmt.setBlueBufferSize(6);
QOpenGLContext *oglc = new QOpenGLContext;
oglc->setFormat(ogfrmt);
oglc->create();
qDebug() << "QOpenGLContext red buffer size:" << oglc->format().redBufferSize();

When digging into the code, I found out that in the function qcgl_createNSOpenGLPixelFormat() the color depth nor the alpha depth is set.