Dwayne Kellum’s video “High Altitude Balloon 24 (I_See_Stars!)” shows the curve of the horizon. This is visible when the horizon is both above and below the center of the frame.
The camera is a Sony A7s II. The Lens is a Sony FE 50mm F1.8.
This lens is very rectilinear. Here is the quantification of the rectilinear properties:
This grid shows the non-fish nature of this lens. A fish-eye lens or one with strong barrel distortion causes straight lines to curve. This lens does not have such a problem. It does not matter if a line is above or below the center of frame. There is not induced curve from the lens.
Here is a screenshot from Dwayne’s video at 117,431 feet, the yellow line is added to see the curve:
Here is the same screenshot compressed to 10%: