Glad to say that the lastest Nikon firmware update 1.20 now has fixed this problem in both the Z6ii & Z7ii. Time to start shooting again in my favourite picture profile.
Yeah its a bit of a pain and so glad I did tests before a job. The footage looks fine when using the playback function on the camera. It's just when you start looking at it on a computer. Tried Davinci, Premiere and even Preview and they all show the flicker.
I have the same problem. Reported to Nikon and they they asked Blackmagic to look into it as I use Resolve. Neither could tell me what was going on there. All the in camera Nikon picture profiles work fine.