[Ardamir the Blessed]

Yes, it was. But my point was that there are actual hints in the Akallabêth (and 'Aldarion and Erendis') that the world was round before the Downfall, even though most readers seem to think that according to the Akallabêth the world was made round at that point. But I now think it clear that it was written in a Round Earth context; even Elendil who wrote it seems to have known that the world was round before the Downfall:

Sauron with many arguments gainsaid all that the Valar had taught; and he bade men think that in the world, in the east and even in the west, there lay yet many seas and many lands for their winning, wherein was wealth uncounted. And still, if they should at the last come to the end of those lands and seas, beyond all lay the Ancient Darkness. 'And out of it the world was made. For Darkness alone is worshipful, and the Lord thereof may yet make other worlds to be gifts to those that serve him, so that the increase of their power shall find no end.'

It also seems that a condition that the Númenoreans would worship the dark (Melkor) is that they think that the world is flat, because then they think that there is an Ancient Darkness where the Lord of the Darkness dwells.

In the passages concerning the actual Downfall there is no explicit mention of the world becoming round either:

But Ilúvatar showed forth his power, and he changed the fashion of the world;

And then after the Downfall the Númenóreans discovered that the world is round:

And those that sailed furthest set but a girdle about the Earth and returned weary at last to the place of their beginning; and they said:
'All roads are now bent.'
Thus in after days, what by the voyages of ships, what by lore and star-craft, the kings of Men knew that the world was indeed made round

So I think that the Númenóreans, before the coming of Sauron to Númenor, knew that the world is round, and this is indeed stated in the Drowning of Anadûnê (for example Meneldur by starcraft could probably see that the world is round, just like the Númenóreans did after the Downfall, see my quote above), but Sauron lied to them saying that it is flat. Elendil was not (at least not much) affected by Sauron's teachings so he did not receive a wrong conception of the world. Then after the Downfall the Númenóreans (of which almost all should have been of the Faithful, but still they seem to have been subject to Sauron's lies) rediscovered that it is round, but they seem to have thought that it was made so in relation to the Downfall (except at least Elendil). But I do not know why the Gates of Morning, which signify a flat Earth, are mentioned in the Akallabêth:

they came even into the inner seas, and sailed about Middle-earth and glimpsed from their high prows the Gates of Morning in the East

Tolkien introduced that passage in the Akallabêth from the Fall of Númenor III, which I believe was written in a Flat Earth context. According to 'The Drowning of Anadûnê' in Sauron Defeated the Akallabêth is a mix of Elvish and Mannish tradition, and it seems that Elendil thought that there is a Gates of Morning in the east, even though he knew (it seems) that the world was round already before the Downfall.