Camera settings in bright sunlite

I am using DD for NDVI analysis on agricultural crops in Australia. I am using a DJI P3 Pro. When filming or photographing at this time of year I need to use an ND filter: 16/32 or even 64 to cope with the brightness. I can then correct in post without having washed out areas.
With the DD flights I have tried just using an ND 4 filter, but I get this “spotlight” effect of a small circular area that is overexposed in each photo. Is this just an effect when using the Phantom’s RGB camera under these conditions? Any advise welcomed.