In The Rings of Power, there are many pockets of characters who come to reckon with the portents of a changing world, not knowing that the reason behind those portents is the rise of evil. It won't matter until the end of Rings of Power. Middle-earth is supposed to be a flat plane. It had been for a long time.
One world in a larger universe called E was created when the God of the Ring, Eru Ilvatar, and his servants in the Ainur created the world. The period of time Rings of Power is set in is thousands and thousands of years long.
You might wonder what makes it turn not-flat. After the Nmenoreans captured Sauron, the Dark Lord told the Elves that there was a Ring to rule them all. The Nmenorean and their king, Ar-Pharazn, take Sauron back to Nmenor and Sauron does what he does best.
The Nmenoreans were tricked into turning against the Elves by Sauron, who convinced Ar-Pharazn to launch a fleet to invade Aman, home to the Valar, their servants. In his fury, Eru Ilvatar destroyed Sauron's physical form, as well as Ar-Pharazn's fleet and Nmenor itself. He made Aman impossible to sail to except for the Elves by making Arda a sphere. I'm extremely angry.
It is only then that Arda is a spherical planet. It might happen in Rings of Power. We know that Nmenor is a major player in the series and that's why we're able to see it. Middle-earth has a vital part to play in the downfall of the Nmenoreans. Galadriel and her fellow warriors will get an all expenses paid trip to Valinor if they don't find the evil she sought in Rings of Power. It is not possible for anyone other than the Elves to make the journey to Aman in Rings of Power, and the show simply decided that a casual audience didn't need the wild lore that is "God gets so heated he makes the world round." They can say it happened at some point and move on, or never get into the whole is-the-world-flat-or- not thing at all.
With the amount of money Amazon has sunk into Rings of Power, I would have expected them to put it on the screen.