I worked it out. It was because the QOpenGLWidget uses an FBO under the hood, and so all calls get redirected to it. I guess Qt doesn't then do any special mapping of FBO color attachments to display buffers to allow for GL_BACK_LEFT and friends to continue working seamlessly.
Solution was to use a QOpenGLWindow instead, and I'm hoping that QWidget::createWindowContainer() will let me continue to work as expected.
