below is from Nikon support:
"I tested our Z6 outdoors today, it was bright out today and even more so with the higher ISO and slower shutter speed requirement. I didn't have ND filters to use, but I had some degree of contrast at 1250 at least, and I didn't see a hint of flicker, even in the waveforms. Can you test this outside and still see an issue, the sample you sent were shot indoors. Maybe you have a bad cable or bad port or bad connection?
I have not received any other similar reports at this time. If you want to have use test the camera here, please let me know. "
his logic is simple: you have issues for indoor shootings , but I have tested, no issue for outdoor shootings.
if you shooting moving subjects, overexposed, maybe not easy to notice this issue.
then what I can say? he is a very smart guy.