I mentioned the paper "A Puzzle about Further Facts" (Erkenntnis, 2019, https://link.springer.com/article/10.1007/s10670-018-9979-6) in the comments above. I think the most relevant passage from that paper is the following. (The context here -- "Case B" -- is that the protagonist, Fonda, is seeing a simulated world through the perspective of a creature, Alpha, but has forgotten that this is the case, putting her in a similar epistemic situation as ourselves. The question is whether she is right to conclude that there are further facts, i.e., that a different perspective, say Beta's, could have appeared to her, and therefore that there is something else to know about the world, like the flag that you mention, with all the consequences thereof.)
Certainly it is true that a different agent’s perspective could be made to appear to her; all this would require is a change to the code governing which perspective is displayed by the VR system. But would she be justified in believing that a different perspective could have appeared to her? Again, it seems that the moment we allow her to remember even just the mere fact that she is in a simulation, even if she remembers nothing else about her identity in the outside world, she can indeed conclude that a different agent’s perspective could have been made to appear to her. But we explicitly rule out such a memory in Case B. Hence, it is not clear that the notion of a different perspective appearing to her makes sense from Fonda’s perspective. For all she knows, she is Alpha, and how could any perspective other than Alpha’s appear to Alpha?
While the author states that this kind of reasoning seems the best way to avoid concluding that there exist further facts (= additional flags) if one seeks to avoid that conclusion, he does not endorse such reasoning either, saying the argument is "in need of further fleshing out" and explicitly leaves open the possibility of concluding that there are indeed further facts.
Also relevant is the paper "Against egalitarianism" (Analysis, 2013, see also https://en.wikipedia.org/wiki/Vertiginous_question) in which the author (Hellie) argues:
The Hellie-subject: why is it me? Why is it the one whose pains are ‘live’, whose volitions are mine, about whom self-interested concern makes sense? That thing there in the objective world: what is so special about it? Why doesn’t some other subject of experience there in the objective world ‘go live’ in this way: for instance, the ‘Chalmers-subject’ out there driving around in the human being whose visage matches a photo on a certain driver’s license bearing the name ‘David Chalmers’—why not instead it?
This "vertiginous question" by Hellie also seems to pose a challenge to "open individualism" and related theories, because those theories still doesn't seem to answer the question: they do not make any deep distinction between (e.g.) Hellie and Chalmers.
As pointed out in the comments by someone else, this question:
Why am I this particular human being?
is closely related and there are a lot more references there.