The HDR mode intends to improve dark scenes pictures, but due to the global dimming algoritm that Samsung implemented it is impossible to see HDR movies with dark scenes.
I have disabled all image post processing, switched all ECO modes, but it still dimming on dark scenes. Dаmn, its drives me crazy. Especially you can see that when you open setting menu over dark scene. You can see how it increases the light because the menu is white, but once you close it it decreaces it and again everythig is dark.
Going to return this tv.
Had the same problem.
I decided to turn off the "Auto Detect Lighting" and "Eco Settings".
Contrast set at 18-22; backlight 40, brightness -1/0.
And turned off HDR on TV box. I am satisfied with the result.
The changes that you did will not prevent from global dimming. You will still get it even in SDR mode. And i think samsung will lnever fix or provide option to disable it. This dimming was introdused intentionally to downgrade value of Series 7 and make series 8 more attractive.
This option enabled only for hdmi inputs and compensate rgb range for old external devices. It has nothing to do with backlight dimming and this option disabled for smart apps and tv signal.