I do not know if this happens with the Q80T/Q90T - if someone have it please test the case below and report:
Steps to reproduce:
Picture mode: Movie / Standard / Natural (for me Movie mode)
Contrast enchancer: OFF
Local Dimming: High
Source: doesn't matter - Netflix, HBO, PC, Console
1) Pick a scene, or several scenes from a movie ( 1 mixed with some dark and bright details and 1 purely dark scene).
2.1) If you are on the mixed scene when you start increasing the Brigtness from 27 > 50 - observe the following things:
a) Highlights will get brighter as expected.
b) Dark colors like - brown, dark green, red, dark grey will beocme darker and the details will be CRUSHED.
2.2) If you are on the dark scene when you start increasing the Brightness from 27>50 - observe the following things:
a) Small highlits in the dark will get DARKER !?!?!?!??.
b) Possible black crush and loss of details.
I have almost the exact same problem as you do. I couldn't find anyone else that has it until now. Did you find a solution?
I have the Q70r running firmware 1371. Issue started a long time ago with much older version of firmware.
My problem is only on movie mode and without HDR (standard mode does not have this problem).
How to recreate:
Play non hdr source
Local dimming high
Brightness (it's called backlight on my version but it was changed with the newer model of tv) max.
If brightness (backlight for me) is decreased to ~25-27, colour is fine but if increased, the blacks are messed up.
If i change to standard local dimming, no issues even when brightness is max.