DougJensen

Videosoul is correct, this is very strange.  I spent about 2 hours yesterday matching all the settings on your camera that I could see in the viewfinder and trying to recreate your problem.  No matter what I do, VND functions normally.  So, I then reached out to one of the top techs at Sony overnight to see if he could figure it out , but he's stumped as well.
Here is our advice:
STEP 1:  First of all, make sure your camera is running the latest firmware, which is V4.0. If it is not, then you can download and install it here.
https://www.sony.com/electronics/support/interchangeable-lens-camcorders-ilme-series/ilme-fx6v/software/00259043
STEP 2:  RESET the camera:
Menu > Maintenance > All Reset
STEP 3:   Load my custom ALL File.
After I matched all the settings on your camera (at least the ones I can see) I saved an ALL File.  You should download my ALL file via the link below and load it into your camera.  It will be called "VND-TEST1" when you see it in the camera's menu.  After you have loaded the ALL file, check to see if the VND functions correctly now -- it does on my camera with those settings.   If you still experience the same issue then there is probably something wrong with your camera.
ALL FILE:     https://vortexmedia.cimediacloud.com/r/PIFxEvRjExYn
Disclaimer:  Just to be clear, none of the settings saved in the "VND-TEST1" file are settings I would ever use myself except for 4K XAVC-I.  So, if anyone else is reading this thread, I do NOT recommend the settings in this file. I'm just trying to troubleshoot Nailer's problem.
STEP 4:  Report your findings! 🙂   Good luck.

Well, that's the kind of thing that sounds good on paper, but usually turns out not so good in practice.  There's no way of creating a better REC709 scene file than S-CINETONE for the FX6 because there are virtually no paint menus to make it with.  Not that I think S-CINETONE is very good.  The lack of paint menus is probably the second biggest shortcoming of the camera. 
As for baking-in an MLUT, I've never seen it done successfully.  I'm not saying it is impossible, but I would have to see some actual real-world video that shows a custom MLUT in practice.  Until then, I'm a skeptic.  🙂

Hi Jiri, I agree completely with your comments.  I have made this suggestion to the Sony product managers several times, but it is unclear whether we will ever get our wish or not. But you never know.

I just coated my 200-600 and 70-200 in black over the holidays.  I sure am liking having most of that ugly beige covered up.
BTW, the A1 is a great camera, but for wildlife video, it is definitely a big step down from the FX6.  Every time I take my A1 out so I am lighter and more nimble, I end up regretting some missed opportunities that the FX6 absolutely would have gotten.  No ND, no picture cache, non-movable EVF, are major shortcomings.  
Just my 2 cents on the subject.

When I'm shooting with my F55 or FS7 I always shoot S-LOG (or RAW) and then grade in Resolve. Almost no exceptions to that workflow with those two cameras.
However, after doing a lot of controlled testing and experimentation with my FX6, I feel that I generally prefer the results I get when I shoot with S-Cinetone and then do a final grade in Resolve.  Now, someone might argue that if I'm going to run the footage through Resolve anyway, I might as well shoot with S-LOG3, but I've done my testing with FX6 footage, and I actually prefer grading S-Cinetone footage rather than S-LOG.
I know that S-Cinetone is promoted as being a WYSIWYG shooting mode that shouldn't require grading, but in my opinion, S-Cinetone can really benefit from some grading in post.  Here's a split screen demonstration I uploaded to my Youtube channel that shows the difference between Ungraded S-Cinetone and Graded S-Cineteone.  I prefer the images on the right.