It was debunked well before the Middle Ages.
Anyone who saw a ship coming over the horizon (the sails would appear before the ship itself!) would know that the Earth wasn’t flat.
True. But a man sees what he wants to see and disregards the rest.
The flat Earth as we know it may be a modern myth. The few ancient maps and map-like inscriptions which survive from the ancient world may or may not have been in reference to a flat Earth. The oldest known one is from Mesopotamia, but it isn’t a purported map of the entire world.
The idea that most people thought there was an edge of the Earth that one would fall off seems to have originated *after* Columbus, perhaps as late as the early 19th century, and probably as part of an effort to make him look greater than he was.