That’s interesting to hear. To be honest the behaviour of clearBuffers=False is never known in advance (but I haven’t seen this version of events before). OpenGL doesn’t specify what is in the buffer that gets returned by a flip command; sometimes it is the previous front buffer (as in literally flipping 2 buffers) and sometimes it turns out to be a copy of the previous back buffer (more useful but not consistent with the concept of “flipping”).
I have no idea what would cause you to have quad buffers enabled, especially on a built-in intel card but, basically, they’re free to do what they like. I wouldn’t expect that to be consistent from one machine to the next, so don’t rely on it.