Since Samsung doesn't want to take care of this, I tried it out a bit. All scenarios were tested without eco modes. Color space RGB full 10bit color depth. I set the resolutions in the NVIDIA control panel in the PC category.
My setup:
PC->Q800t (75) HDMI 4 -> HDR 4k 120 hz no game mode local dimming is broken.
PC->Q800t (75) HDMI 4 -> HDR 4k 120 hz Game Mode VRR local dimming is broken, the colors and contrast also look really bad.
PC->Q800t (75) HDMI 4 -> HDR 4k 100Hz
Started the timer again, what do we have here? The 6 minute mark is exceeded, now it switches to standard mode after about 7:20.
For me a half workaround and no solution.
It would be great if someone from the support team could get involved here to get the whole thing rolling again somehow.
Either it is still a software bug that should be patched urgently and long overdue or cheap panels have been installed that are overloaded at 120/100 hz and shut down non-essential systems 
A little update from me, because as an it guy, this doesn't give me any peace of mind.
The assumption that Samsung has to deliver an update here is clear, no question.
But for me I have now found a
workaround to be able to use the whole thing properly on the PC.
I have changed the input source on the TV from PC to games console. Then set the 4K 120HZ. The advantage here is that you have access to the color settings as well as the contrast enhancements and the color space. I think this is blocked in PC mode for reasons of latency (but I don't notice anything here and I do a lot of gaming on good monitors 
).
Then HDR activated everything so far so good. I turned off contrast enhancement because the colors wash out too much for me. I turned up the SDR brightness in the Windows settings and it works fine. Now the usual procedure, HDR on and off and the timer on. Local dimming on high, mind you. after 10 minutes no reduction in brightness. so far so good, now switched to game mode which is unusable if you set the input as PC (Contrasts do not match and colors are also washed out).
VRR and HGiG activated in game mode. Local Dimming on high and contrast enhancement off. Waited 10 minutes and again no dimming.
So i tried everything again with contrast enhancement and what do we see? The error is back.
As a PC user, my recommendation:
Contrast enhancement off, I have tested a lot with HDR content and with a PC as the source, the content is simply distorted with contrast enhancement.
1. set input to game console
2. game mode on
3. color space on auto
3. activate 4k/120 Hz HDR
4. activate HGiG
5. deactivate contrast enhancement
5. activate VRR
6. set SDR slider in Windows to personal reference (I have it at 100, no washing out or something, looks like SDR)
7. run HDR calibration tool and be happy
8. please report if it works for someone else.
PS: I have set the sharpness on the TV to 0, at least I don't notice any artifacts here with text etc.
Regards
Hello again,
Technician connected remotely today and reset the EDID.
Local dimming High, contrast enhancer High as well as 120hz and VRR in console mode (PC is connected) no dimming even after 10 minutes.
Why in console mode?
In PC mode, the setting for the color space and the contrast enhancer (only in game mode) is greyed out, which makes the whole thing unusable.
So for all those who have a console, pick up your phone
If there is now a FW update that cleans up the PC mode, I'll be happy.
I showed him our post here while he was connected and he said he didn't know that at all ... He'll pass it on, let's see what happens. They want to get in touch again on Monday.
