Some people are confused that a full moon is visible all night, and think if the full moon is the result of the opposition of the moon from the sun, then a full moon should be visible only at midnight.
Flat-Earthers who have such spatial visualization problem take a step further and use it as ‘evidence’ that the Earth is not a sphere. They are wrong. The full moon phenomenon can easily be explained in the spherical Earth model.
Such confusion is normal. Not everyone has the same spatial intelligence. And it should not be too hard to show them how they got it wrong.
Usually, people who get confused are too fixated on the position of the sun, moon, and Earth relative to each other; but did not pay enough attention to the position of an observer standing on the surface of the Earth. An observer on Earth has a clear view of the sky in every direction, not only the sky directly above. Any object above the horizon is visible.
However, flat-Earthers are actively looking for ‘holes’ in mainstream theories and would use their misunderstandings as ‘evidence’ against accepted facts. In this case, they would use the fact they don’t understand how full moons appear as ‘evidence’ against spherical Earth. They are obviously wrong, even if it can be much harder to show them how they got it wrong.