I'm also facing the same. At first I wonder why the footage become blurry. Thinking the lens was not clean I landed and checked and the lens was foggy in the centre area. After a min the fog disappeared but as soon as I start flying it again started. I was upset and checked YouTube and find that many are facing the same as I do. DJI should checked in fixing this.

I had the same issue with my mavic mini. I did a simple trick, I applied some shampoo and wiped the camera before flying every time. This might be silly, but it did work for me!

DuminduM Posted at 9-17 07:37
I had the same issue with my mavic mini. I did a simple trick, I applied some shampoo and wiped the camera before flying every time. This might be silly, but it did work for me!

Really man? I works? Do you apply shampoo everytime before the flight?

Hey, I've had some issues with lens fogging. I live in Ireland where it rains and is humid. Every time I captured video, it looked foggy and I noticed fog inside the lens and became unworkable.

Things I learned.
1. I bought NV filters and when they were I didn't experience fogging. Probably because the cold is hitting the filter and not the lens directly, but it's a non tech fix.
If your lens is fogging,  pick up a good set of NV filters on Amazon and see does it help. They also really help when flying early and late in the day, when sun glare is problem, I don't fly without filters any more.
2. I went ahead and bought a new lens and replaced it. There's plenty of guides on YouTube. Remove 6 screws and 2 little covers. At this point my lens would not come off.
In the end I got a pliers and covered the metal with a cloth to protect it and gently pulled, this worked. I then observed, that it  was  a paste, like a heat sync that was holding it.
I finished replacing the lens, went for a test fly and compared the footage to my old footage and it was unmistakably crystal clear in comparison.

Summary
My Mavic Pro 1 is about 3 years old. I can't account for the first year because I bought it 2nd hand. I actually think it was sold to me like this and was foggy from day 1, but I didn't know because it was my first drone. I think if the Mavic is flown is humid, wet conditions over time, moisture can get into the lens and cause issues.
If it is affecting you and you don't want to replace the lens, try the filters.
If this issue persists, you've nothing to lose, get onto Ebay and buy a new/used lens.

I bought a new one and if I come straight from the house and turn it on, with no NV filter on, even the new one will fog.
The NV filter is a good protector for the lens and improves video quality too.
That's my experience, been all good ever since.

Darragh Posted at 2020-10-3 03:33
Hey, I've had some issues with lens fogging. I live in Ireland where it rains and is humid. Every time I captured video, it looked foggy and I noticed fog inside the lens and became unworkable.

Things I learned.

I'm in Northern Ireland i bought my Mavic new and its had about 10 flights. All the footage is useless due to the fogging. I fly FPV more and don't have any fogging issues with my Rotor x Atom, TBS Discovery or 250 sized quads. Its ironic that the one expensive drone i bought for filming can't be used for that purpose. I got the usual DJI support 'send it in for repair but its out of warrenty'  response.  Found this thread and hoping the filters will make a difference, thanks.