DIAGNOSIS This is likely the Dolby Vision precision detail processing interacting with the app's streaming profile. The mismatch causes momentary brightness fluctuations, often called "near-black flashing" or "Dolby Vision flicker."
FIX First, change the Dolby Vision picture mode. While playing the Dolby Vision content, press the Picture button on the remote. Change the mode from Dolby Vision Bright or Dolby Vision Dark to Dolby Vision Vivid. This uses a different tone-mapping curve. If that helps, go to Settings > Display & Sound > Picture > Adjustments. In the Dolby Vision mode, set Reality Creation to Off and set Random Noise Reduction to Off.
IF THAT DIDN'T WORK The issue may be specific to the Disney+ app's implementation. Force stop and update the app. Go to Settings > Apps > See all apps > Disney+. Select Force stop, then Update. If no update is available, select Clear cache. Unplug the TV from power for 60 seconds to reset the video processor.
HEADS UP This is a known software-level issue with some Dolby Vision content streams, not a hardware fault. It may be improved or resolved in a future firmware update from Sony. Check for updates periodically.