Apple TV 4K displays flashing white box
Apple TV 4K blank white box bottom of screen flashes a word then back to white
Apple TV 4K (2nd generation)
Question marked as
The "flashing word" is the key clue. Your Apple TV is correctly trying to show you dialogue, but the visual settings for how that dialogue looks are broken (likely White-on-White). Changing the Subtitle Style to "Transparent Background" usually solves this instantly.