This system has two monitors and reproduces the problem as described. Two other systems with only one monitor show slightly different symptoms after the last xrandr --mode call: the screen freezes or stops updating while the mouse pointer is still responsive. I can still move windows around and by the shape of the mouse pointer I can tell that I did move the window. But the image on the screen is not getting updated.
This system has two monitors and reproduces the problem as described. Two other systems with only one monitor show slightly different symptoms after the last xrandr --mode call: the screen freezes or stops updating while the mouse pointer is still responsive. I can still move windows around and by the shape of the mouse pointer I can tell that I did move the window. But the image on the screen is not getting updated.