Looking at the OP footage again.
***
Slow and steady shape-shifting through 0:18-0:38:
(1) The camera shakes and zooms
more violently than the slow and steady shifting of shapes.
(2) The sharpness of the image seems to remain steady
despite camera shaking and zooming.
Correct me if I'm wrong, but it would be very difficult to retain such steady sharpness
during focus shifts. Also, assuming it's 'slow focus shifts' causing the 'slow shape-shifting', it would be very difficult for the camera to perform slow focus shifts
during violent camera shakes and zooms and seamlessly continuing these shifts when the violent camera shakes and zooms suddenly stop.
Steady elongated Y-shaped pattern through 0:54-1:02:
(1) The elongated Y-shaped pattern remains quite stable
despite the camera shaking and zooming.
(2) Here also the sharpness of the image does not seem to change
despite the camera shaking and zooming.
Again, focus shifts would likely result in sharpness changes and be affected by the violent shakes and zooms by the camera and their sudden cessation.
***
In the
first video shared by
@MclachlanM (also shot by Nikon P900 from plane), the shape of the image is indeed very similar to the OP footage. Therefore, the model and make of camera equipment seems definitely to come into play in explaining the still refraction. However, the shape in his video doesn't change as demonstrated by comparing the zoomed image at 0:21 and at 1:04. This seems to rule out window/lens refraction, atmospheric refraction as well as the movement of the plane (alone or together) explaining the shape-shifting, and adding to the strangeness of the OP footage.
However, if the above process of elimination of focus shifts, window/lens refraction and atmospheric refraction withstands critique, the hypothesis of
a dynamic refraction created by a droplet of water acting together with a constantly changing angle due to the movement of the plane gains in credence.
It could also be tested by someone with the right equipment and 'lab' conditions -- a point source of light, Nikon P900, water and a steadily changing camera angle.