Panorama by shooting straight down and moving P4P?

[Meta4]

But in my case, the camera is pointed straight down and the drone is moving horizontally on a grid, taking overlapping images. There is not one subject (green box) being photographed. And as such, there is no angle to the subject as demonstrated in your diagram. The resultant images should be no different than photocopying a large book by creating multiple images with just parts of the book on the glass and then assembling the images. All images are taken from the same angle, perpendicular to the flight path with overlap. It should be the simplest stitch imaginable.

The stitching software is looking for common points to match in pairs of images.
The lens is pointed straight down toward the centre of the image but the angles from the centre of the lens to the points the camera is trying to match are all over the place.
The simple test to see if it's a factor would be to try a few at different heights.

In orthophoto mapping missions when I have trouble stitching, the solution is to go higher and/or increase overlaps.

 

[bsartist]

If the subject is not flat (I.e. if there are trees or buildings) you could end up with parallax distortion as the relationship between things closer to the camera will change relative to the ground.