According to Round Earth Theory the sun is illuminating the earth and moon as in this top-down view:
However, it has been observed that during the day, on the "day" side of the earth, the moon can be seen with phases which show significant lit area.
It should be impossible for the moon's phase to display significant fullness during the day. But the moon is seen on many occasions to do just that. Here is an example which should be impossible under the Round Earth model:
In the video the author pans across to the sun, showing that it is significantly above the horizon. This person is clearly somewhere on the day side of the earth. How is this observation possible?