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:

https://www.the-digital-picture.com/Reviews/Lens-Distortion.aspx?Lens=1195&LensComp=0&Camera=1175&CameraComp=0&FLI=0&FLIComp=0&Grid=N

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%: