My new D800 has Left/Right AF issues too :(

I would first suggest to:

1- Do your test with daylight

2- Stop losing your time with flat targets. They are worthless and the best thing to use to send any camera back to the manufacturer. Instead, fine tune your lens for the central AF sensor at a distance of 50 times the focal length, as documented here

https://nikoneurope-en.custhelp.com/app/answers/detail/a_id/51633/~/how-to-use-the-af-fine-tune-function

(I would suggest to shoot 5 times for every AF Fine Tune value, and step by values of 10 (-20, -10,  0, 10, 20) then by values of 5 close to the good one).

Then do the same thing for the left and the right AF sensor. Then, post the results.

If the difference is less than 10 in between those numbers, then your body should be considered to be fine.