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it's not even a real opencv problem --

ROI's are just numpy slices, if you modify them, you modify the original image data, no need to copy anything back, see:

>>> a = np.zeros((200,200,3),dtype=np.uint8)
>>> a[80:120,80:120,:] = (127,127,127)
>>> cv2.imshow("A",a)
>>> cv2.waitKey()