The ancients knew the earth was spherical. Everyone who knew enough to know anything in the Middle Ages knew the earth was spherical. The objection to Columbus was not that the earth was flat, but that he had miscalculated how far it was to Asia going that way. And he had, in fact. Columbus was wrong and the people who told him he had miscalculated were right. In fact he didn't reach Asia and couldn't have anyway. Remember this was a great age of navigation, and he was not the smartest navigator out there.
So the flat earth myth is not that the earth is flat, but that people thought the earth was flat. So this is bad history of science in the service of the promotion of science. One of the main perpetrators of this was Washington Irving. One of the original Hispanophiles in the US, he wrote a biography of Columbus. His little factoid wormed its way into schoolbooks, and I grew up thinking it was true.
Of course, probably there were some medieval people on the farm who might have thought that about the earth, to the extent that they thought about this at all, but this was never official expert knowledge. Ever.