This is most likely heat haze/atmospheric shimmer, which becomes very visible at extreme focal lengths. Even in full daylight, small temperature differences in the air can distort parts of the scene unevenly, creating those patchy “watercolor” blur areas while other parts still look sharp.

At 1000–2000mm equivalent, you’re magnifying not just the subject but also the disturbed air between you and it. High-altitude, sunlit conditions can make this worse depending on the ground and air layers you’re shooting through.

Noise reduction is unlikely to be the main cause. The camera’s processing may make the effect look a bit stranger, and the P900 may also be applying distortion correction internally, but the root problem is usually the air, not focus or JPEG settings.

To reduce it:

• get closer to the subject
• shoot through less heated air if possible
• change your angle or elevation
• wait for steadier air conditions
• avoid the very longest zoom settings when possible

If other images in the same session are sharp, that also supports atmospheric conditions rather than a camera fault.