Hi Nickname,
There are a few possibilities, please try to adjust the below Settings to prevent this from happening:
First, you may want to disable Power Saving Mode, which automatically adjusts the brightness to reduce the TV’s power consumption. Go to Settings > All Settings> General > Eco Solution> Power Saving Mode.
Next, Auto Game Mode might be affecting the brightness. To change this, go to Settings> All Settings> Connections> Game Mode Settings.
I hope this helps!
~Samsung Advisor