I solved the propbem.
1- Go to Samsung service menu
2- Go to [Option]>MRT Option>
3- in [MRT Option] change the following tab to 0
4- Set [ECO Sensor] > 0
5- Set [Locak Dimming]> 0
6- Turn off the TV
I upload a video to show how it's done.
It’s crazy that in such a premium tv, “an OLED killer” one there is this issue.
I was thinking to return my Q7FN and pay the extra for Q9FN, but now that you say you have it as well this is becoming a joke.
Samsung - the case is very simple, dark room, dark scene and subtitles on. You will see the screen getting seriously brighter when subtitles are displayed and then darker when they are off. Crazy.
Have there been any changes to Picture settings? For testing purposes, can you reset the Picture to the factory settings? To do this follow the directions in the link.
Has the TV's brightness always changed when the subtitles are turned on? Also, have you tried replacing the HDMI cables for the TV? It is possible a defective cable is causing the symptom.
HDMI cable isn't the problem, as this happens with all sources, including apps in the Samsung TV app store (Netflix, Prime Video, Youtube) and same apps on the PS4. It definitely is an issue with the firmware.
There is not a HDMI cable problem because I have this problem with all the content, especially with HDR content via usb with the internal mediaplayer, netflix, youtube...
Samsung, please treat this with a priority. This issue is being reported in every Samsung/QLED forum I visited, formal and informal. It is related to the Local Dimming implemetation and has nothing to do with the source/cables or anything else.