What could be the problems is that your Samsung TV is set to automatically use Filmmaker Mode when watching a movie. Although restarting has no effect on this setting. You will see this when the UI of Apple TV is bright as usual, then once the movie starts, the content is too dark. You have a setting on your TV to prevent it from using this Filmmaker mode.
Each model of TV may have the setting in a different location, but on some Samsung TV's, they offer this path:
Press the Home button > Select Settings > Select Picture > Select Expert Settings > Select Film Mode > Select Off