T.S

Update:

updated my Z6 to the new firmware V2.0 today, flickering issue still there..no fix at all.

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.

"and flicker or noise may be visible in the display." , but actually the flicker is not only visible in the display but in the final unusable videos as well, 

tried all the options, no help. I did the test under totally natural light.

flickering at certain ISOs, like 1250, 4000,5600 etc, other ISOs are ok. 

happens with N-log/10 bit, all settings and different cables  were tried still no solution.

please refer to the video I tested.

anyone has the same issue and any solution?

PS, just updated the firmware to V1.01, flickering issue still there.

you can press the ISO button once, then adjust the ISO, don't have to always pressing the ISO button