Mavic Pro with the latest firmware, iphone 7+, RC with the latest firmware 1080p 30 fps, Manual mode, ND4 filter, Iso 200, Shutter 100. SD card from the Mavic Box The weather is cold and foggy (it's just a flight test) look at the trees and their foliage on the raw footage HERE (direct from SD, just cut, because it's a big file). The car is fine, but the trees ... is it an AF issue ? Any thoughts ?

Try setting sharpness to +1 which will disable noise reduction (which can cause the artefacts in your video).

Did you Tap to focus or manual focus?

Tap to focus... Perhaps, i should use continuous focusing ?

too dark ,  exposure at the the limits of the camera, wide open aperture results in shallow depth of field, which means a lot of things look out of focus.

As you increase the aperture to increase the depth of field the iso speed has to compensate , which then introduces digital noise which the again affects sharpness

It's a Mavic, we can't adjust the aperture... and the ISO is manually adjusted.

Like JBInspire said, try to set sharpness to a value needed to produce a sharp image. I leave sharpness at -1 when not using an ND filter and having a high shutterspeed since I otherwise have an oversharpened image, but when using an ND filter with low shutterspeed (like 1/50th) I need to put sharpness at +1 to not have a washed out picture. Just tinker around and watch it at home on a big screen.
Good luck!

There is nothing wrong with your mavic in my opinion, the footage looks similar to results I get in conditions like this.... remember, the mavic camera is basically a cell phone camera so you might be expecting too much...

my Iphone 7 plus give me much better video (but it don't fly alone)... I'll try to set the sharpness at a higher level next time.

Horrible light = horrible video, and the Mavic's camera really wants to have lots of light. Welcome to what's colloquially known as the watercolor effect -- you often see this in low contrast, underexposed lighting.

As several have noted, setting sharpening to a positive value can help, but most often we see this when people use a negative sharpness value in poor lighting. I never find a need to use +1 in my part of the world, and am tempted to use -1 when in really bright scenes, but using -1 in poor lighting is a recipe for disaster; your mileage may vary, etc.

Another thing that can help with exposure is to activate the histogram and try to get a more even distribution. This is what your histogram looks like for the opening frame, you want to aim for getting that hump in the middle rather than at one end or the other: