Some people are confused that we can see a full moon all night. They expect 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 with such a spatial cognition problem go a step further and use it to “disprove” spherical Earth. In reality, we can easily explain the phenomenon in the spherical Earth model.
This misunderstanding is not unusual. Many of us have trouble visualizing objects in three dimensions. However, it is easy to explain the geometry to get practically everyone to understand.
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 180° view of the sky in every direction, and not just the part of the sky directly above. Therefore, any object above the horizon will be visible.
However, flat-Earthers actively look for “holes” in mainstream science and use their own misunderstandings to disprove science. In this case, they will use the fact they cannot understand how a full moon can happen as “evidence” against spherical Earth. However, most of them can actually understand our explanation; they are just unwilling to accept it.