Samsung UN43J5200AF

Disable Adaptive Brightness

Picture:
  Picture Mode:        Movie  # <-- **ONLY MOVIE** disables adaptive brithness entirely
  Advanced Settings:
    Dynamic Contrast:  OFF
    Black Tone:        OFF
    RGB Only Mode:     OFF
    Color Space:       NATIVE
    Motion Lighting:   OFF

  Picture Options:
    Color Tone: STANDARD
    Digital Clean View: OFF
    MPEG Noise Filter: OFF
    HDMI Black Level: LOW
    LED Clear Motion: OFF

System:
  Eco Solution:
    Energy Saving: OFF
    Eco Sensor: OFF

See Also https://www.samsung.com/uk/support/tv-audio-video/2022-how-to-troubleshoot-when-the-screen-brightness-of-your-samsung-tv-changes-automatically/