Depth Buffer not working in 5.15.2

The web page "https//doc.qt.io/qt-6/supported-platforms.html" implies that Qt is supported by openSUSE 15.3 and 15.4.  As I am trying to port an OpenGL 2.1 code to Qt I've started with Qt5 that is available on both platforms.  I've tried out a number of tests all of which generated unexpected results - all of which could be explained if the depth buffer was not being used, despite any calls to enable and clear the buffer. 

For this bug report I have gone back to the basic example program given in:
  https://doc.qt.io/qt-6/qtopengl-openglwindow-example.html

The example is a bit of a cheat (bug?) in that it supposedly illustrates a moving 3D shape but does so without using a depth buffer and 'forgetting' to tell the reader of the missing buffer.

However if I change routine TriangleWindow in main.cpp to add and clear the depth buffer and plot a second offset triangle, I find that the triangle that appears to be 'in front' is the one last plotted not the one that should be furthest from the eye.  The two snapshots included show cases where the second triangle should be nearest and furthest from the eye but in both cases it appears in front.

I also enclose a copy of the modified main.cpp.

Regards,  DJW

p.s.  After failure with openSUSE 15.3, I upgraded to the latest stable version (15.4) but the results are the same in both cases.  In 15.4 the Qt version is 5.15.2