When shooting under direct sunlight, the viewfinder image is not clear. This is due to irregular reflection in the viewfinder caused by the light reflected from the surface of the eyelid or eye. It can happen if you are not holding the viewfinder close enough to your eye. It is more common, however, when wearing eyeglasses because the light is reflected through the viewfinder from the gap between the eyecup and eyelid.
Try covering the upper part of the viewfinder with your hand to avoid this issue.