QOpenGLWidget::grabFramebuffer is darker than screen: not doing same gamma correction?

The QImage produced by QOpenGLWidget::grabFramebuffer is much darker than what appears onscreen (see attached images) – seems like some kind of gamma correction factor is missing? It would be much more useful if the resulting image reflects what is seen onscreen – perhaps as an option?

Relatedly, when I recently switched from using the QGLWidget to the QOpenGLWidget, it seems that this gamma factor got introduced, as the same underlying opengl rendering code now produces a much brighter image. I have searched extensively for any info about this but can't seem to find anything – perhaps some underlying OpenGL flag somewhere set globally on the entire window that automatically applies a gamma transform?