Triangles in wireframe are not filled in correctly

While running Qt 3D wireframe example - the trefoil knot shape wireframe is not rendered properly. Some of the triangles are not filled in and the image "blinks".
See attached images.

The problem does not exist in other Qt3d and QML examples.

Run on Linux Ubuntu 18.04.3 LTS, Qt from online installer,
g++ (Ubuntu 7.4.0-1ubuntu1~18.04.1) 7.4.0

ldd wireframe | grep GL
libGL.so.1 => /usr/lib/x86_64-linux-gnu/libGL.so.1 (0x00007f56f1b28000)
libGLX.so.0 => /usr/lib/x86_64-linux-gnu/libGLX.so.0 (0x00007f56ee598000)
libGLdispatch.so.0 => /usr/lib/x86_64-linux-gnu/libGLdispatch.so.0 (0x00007f56ee2e2000)