Let's say you've either cloned yourself a native population (the ethical option) or taken a bunch of volunteers and wiped all their memories of modern technology and society (the slightly more morally iffy option), then taken your fresh batch of blank slates and dropped them into a closed, simulated environment mimicking Earth (in this case an O'Neill cylinder spun up to either full Earth gravity or an appreciable fraction thereof) to see how they develop on their own.
This social/anthropological experiment is going nicely if you, a staff member, can stomach the slightly dodgy ethics of breeding an entire native population for what is essentially glorified roleplaying with the project overseers as the DMs/de facto gods of this neat little fantasy world you've encapsulated. For funsies, you've even genetically altered and segregated the population into three distinct species occupying different "levels" of the cylinder, which is built like an onion. But you, in your godhood, now have a problem you must address.
How do you prevent your new, primitive occupants from realizing they're living in an artificially-made cylinder? Do you come down from "heaven" to tell them the truth yourself, and risk contaminating the experiment? Do you attempt to cleverly hide it somehow by either altering the landscape to hide it or simply transferring them to a larger cylinder where the curvature would be less noticeable? Or do you simply not bother to disturb the subjects and allow them to draw their own conclusions, reacting with amusement as they form entire religions and creation myths to explain why the Earth is curved? Is there a better/best option?