21v DC-DC step down power supply on led driver board offers little current. It works at 440kHz with 22uH inductor. That could give less than 0.3A. It's not enough. While in the vivid mode, dark area becomes darker, bright area becomes brighter, it consumes much more current than normal while the whole screen is white. So the voltage drops and the backlight blinks.
If you use an inductor slightly less than 22uH, it would change the outcome.