If you shoot an image with a large area of bright sky and a smaller area of dark foreground, the camera exposure metering has a problem dealing with the difference in brightness from the two major elemants.
Left to make the decision itself, the camera will expose for an average of the scene which will be unsatisfactory all round.
Some solutions could be to either:
- Expose for what you want to be the subject rather than the average of the whole frame.
- Bring up shadow detail and/or reduce highlights in your post processing
- Compose for a more evenly lit scene
Is there any particular reason you were using the ND filter?
In stills photography, all it will do is lower the light level and force you to have a slower shutter speed.
Unless you want to shoot with a slower shutter speed, there's no reason to be using an ND filter in stills photography from a drone.