It is not difficult to see how the story of Columbus was adapted so that he became the figure of progress rather than a lucky man who profited from his error. According to Jeffrey Burton Russell here, the invention of the flat Earth myth can be laid at the feet of the nineteenth century writer Washington Irving, who included it in his historical novel on Columbus, and the wider idea that the everyone in the Middle Ages was deluded has been widely accepted ever since.

The myth that Christians in the Middle Ages thought the world was flat was given a massive boost by Andrew Dickson White's weighty tome The Warfare of Science with Theology published in 1896. This book has become something of a running joke among historians of science and it is dutifully mentioned as a prime example of misinformation in the preface of most modern works on science and religion. The flat Earth is discussed in chapter 2 and one can almost sense White's confusion that hardly any of the sources support his hypothesis that Christians widely believed in it. He finds himself grudgingly admitting that St Clement, Origen, St Ambrose, St Augustine, St Isodore, St Albertus Magnus and St Thomas Aquinas all accepted the Earth was a globe - in other words none of the great doctors of the church had considered the matter in doubt. Although an analysis of what White actually says suggests he was aware that the flat Earth was largely a myth, he certainly gives an impression of ignorant Christians suppressing rational knowledge of its real shape.

Anti-clerical history of science writers have promulgated the myth so that even today, in his book , Daniel Boorstin manages to produce a totally misleading account (although he eventually gets Columbus right). His bias shows badly when he castigates Christians for thinking the world was flat when they did not and then praises the erudition of Chinese geographers who actually did believe it. The myth is so prevalent that the blurb on the back cover of the UK version of Umberto Eco's book .  He finds all educated people in the Middle Ages were well aware the Earth was a sphere. Perhaps today we can at last dispense with this patronising belief about people who lived in the past.

Usually, even a non-Christian knows something about the earth, the heavens, and the other elements of this world, about the motion and orbit of the stars and even their size and relative positions, about the predictable eclipses of the sun and moon, the cycles of the years and the seasons, about the kinds of animals, shrubs, stones, and so forth, and this knowledge he hold to as being certain from reason and experience. Now, it is a disgraceful and dangerous thing for an infidel to hear a Christian, presumably giving the meaning of Holy Scripture, talking nonsense on these topics; and we should take all means to prevent such an embarrassing situation, in which people show up vast ignorance in a Christian and laugh it to scorn. The shame is not so much that an ignorant individual is derided, but that people outside the household of faith think our sacred writers held such opinions, and, to the great loss of those for whose salvation we toil, the writers of our Scripture are criticized and rejected as unlearned men. If they find a Christian mistaken in a field which they themselves know well and hear him maintaining his foolish opinions about our books, how are they going to believe those books in matters concerning the resurrection of the dead, the hope of eternal life, and the kingdom of heaven, when they think their pages are full of falsehoods and on facts which they themselves have learnt from experience and the light of reason? Reckless and incompetent expounders of Holy Scripture bring untold trouble and sorrow on their wiser brethren when they are caught in one of their mischievous false opinions and are taken to task by those who are not bound by the authority of our sacred books. For then, to defend their utterly foolish and obviously untrue statements, they will try to call upon Holy Scripture for proof and even recite from memory many passages which they think support their position, although they understand neither what they say nor the things about which they make assertion. [1 Timothy 1.7]

Even if everyone believed in FE for a certain period of time in Church history, that doesn't rise to the level of Magisterium, much less a teaching of the Ordinary Universal Magisterium.

In another thread, Meg made the astonishing claim that Catholics before the Reformation believed in FE.  Usually, when I see such claims, they are made by enemies of the Church in an attempt to discredit her.  It is simply not true.

Here is an explanation from James Hannam:
http://jameshannam.com/flatearth.htm

Let me repeat this bit:  "St Clement, Origen, St Ambrose, St Augustine, St Isodore, St Albertus Magnus and St Thomas Aquinas all accepted the Earth was a globe - in other words none of the great doctors of the church had considered the matter in doubt."

St. Augustine explicitly wrote against spreading this sort of nonsense:
If one does not object on principle to Wikipedia, they have a decent article on this citing more sources that show that educated Catholics have always believed the earth is a globe.
https://en.wikipedia.org/wiki/Myth_of_the_flat_Earth

The whole point of that St. Augustine quote is that it matters a great deal.  By destroying the credibility of Christianity, people espousing FE can lead to the loss of salvation.  He is saying that it is disgraceful and dangerous nonsense with eternal consequences.

This matter does not need to be brought up.  It does nothing to further the reign of Christ the King.  On the contrary, it endangers souls.  

How does the flat earth destroy credibility? And....St. Augustine doesn't say anything about the concept of the flat earth destroying the credibility of the Church, or Christianity, or salvation.