As Neil DeGrasse Tyson says, "That stuff is flat."
See, here's the thing. Tyson is wrong. At the purported altitude of the Red Bull capsule, you absolutely SHOULD have seen visible curvature ... not as pronounced as indicated by the fish-eye lens, but noticeable nevertheless. So was Tyson just plain ignorant or was he lying? If he was lying, why lie?
I surmise that he was lying because various amateur outfits have recently gained the capability to send balloons to that same altitude (around 120K feet), and their fish-eye-lens-less video shows a perfectly flat horizon line at eye level, neither of which should be possible at that elevation over a globe. So Tyson has to lie and claim that the curvature should not be visible at this altitude to dodge being exposed by the amateur footage. In a sense, he's actually right that "that stuff is flat", but wrong / lying in implying that it SHOULD be flat at that altitude.
Purely mathematical models demonstrate that at 120K feet, there should be a noticeable drop in the level of the horizon line (below eye level) as well as noticeable curvature. But no such curvature is visible from the amateur footage. Did refraction make that curvature go away also, and somehow boost the level of the horizon? Amazing properties this "refraction" possesses. In fact, with the Red Bull jump, they made a mistake by showing footage from the same camera both while the capsule was on the ground and when it was at 128K feet (with the door open in both cases). You could see that the horizon line was EXACTLY at the same level in both shots. Yet another thing to consider. You know how high those balloons appear to be from 120K feet? Well ... if you were to look from one end of Kansas to another, there should be an earth bulge of roughly the same 120K feet in the center of Kansas. In other words, Kansas should bulge upward to the same altitude as those balloons were flying. Hogwash. No such bulges are evident (by virtue of dropping horizon line) anywhere in the amateur footage from 120K feet.