I just wanted to post an update to talk about my situation, hopefully it will be useful for folks still having the same problem. I bought a new Q80T in late Nov 2020 that had the 1403 software version (as I remember), and had the same issue especially with HDR Netflix content. For example The Crown S04 and many other series and movies streamed in HDR were so dim and washed out. I tried to fix it by changing the default "Normal" picture settings: changed a lot of picture parameters to max including gamma, contrast enhancer etc. - so it was better but still dim and washed out. The weird thing is that it was not happening to all the HDR content. For example the series "Dark" and the silly Eurovision movie were pretty great in HDR before the magic solution (detailed below) came.
I updated the OS to 1460 (which was not offered by the TV software as an automatic update but I downloaded it from the website), also set the picture to Film mode (while keeping the high gamma settings) and now the HDR looks great, I tested with all the materials that were crappy, and everything looks much-much better now. I don't know if it's because of the OS update, or the internal Netflix app got an update, or just Film mode makes other special changes to the image processing that is not accessible directly from the menu... no idea. So I'm very happy now! I want to believe that Samsung realized the issue and released something in the background to fix it. 
I have the Samsung QE65Q95T new TV with the same problem. I tried update the firmware to the latest 1460.9 (Dec 8, 2020) and the picture was improved but only a little, almost negligible.
Few pictures before the update from "Queen's Gambit" , with and without HDR (note: for the Non HDR I set my Netflix account to "standard" not "Premium"... this was the only way, the TV don't allow you to cancel the HDR....👎😞
Again, after the update, there was almost no improvement
Yes, Queen's Gambit is a good test material, however it's a little washed out in SDR as well. I'm not sure it's a 100% valid comparison, but definitely shows something: I made photos about the SDR and HDR representations of the same Queen's Gambit scenes with my DSLR camera using the same manual shutter and aperture settings, the only biasing factor could be that for the SDR material I used my Mede8er desktop media player (this was the only way I could quickly change between HDR/SDR content). For the HDR material I used built-in Netflix app. Settings are definitely different for SDR and HDR, but the basis for both is Film mode.
I tried to catch exactly the same scenes, but it's not perfect. Please ignore red/green crosshatch effect on the screen, it's obviously not the content but some DSLR camera sensor - TV anti-reflective layer interference. At first glance SDR seems better, but HDR also brings the same amount of lightness but in more granual style showing more details on the face and in the background - which would be its purpose.
Hi,
The combination of Gamma and Enhanced Contrast solved the problem, Thank you!
Now not only you, me too is very happy...
To Summarize the solution for all:
1. Disable the Picture "intelligent Mode" to Enable other picture modes
2. Set Picture mode to Normal, Film or Filmmaker Mode. These modes enables the Gamma settings
3. In one of the above Picture modes:
- Set the Contrast Enhancer from "Off" to "Low" or "High" (I recommend to try both settings)
- Set the Gamma from 0 to 2 or 3 (I recommend to try both settings)
4. Enjoy...
Note: As mention in other post, not all HDR titles have this problem. Some HDR title can be perfectly seen with the picture "Intelligent Mode".
A picture I took (with my phone so the automatic color made the picture with blue tone...):
