In TV mode or during watching video using apps like Youtube, when most part of screen becomes black (for example on text or small/dark logo on black screen, credits, etc.) all other colors become dimmed (for example, bright white text becomes gray or even dark gray) .
All possible related settings are switched off, firmware updated to 1403 via Internet, factory reset after update was performed.
I think I found a solution. It looks like there is a bug in Eco Solution settings. Ambient Light Detection works anyway regardless of feature turned on or off.
So, as workaround, I enabled Ambient Light Detection, set Minimum Brigthness to the same level as the brightness in Picture settings, then disable feature again (not sure if it matters).
Not a 100% solution
Dimming happens anyway (but less significantly), especially when Local Dimming set to Medium (not Low or High). It looks like this is another bug (or still first bug with Eco Solution).