Switch to fullscreen and 4K (even if you use a HD display), look at the lower left area of the video - there's quite some strange effects, like darker/different colored patches dancing around. Subtle, but visible.
I noticed that effect with HD 4K videos showing more or less plain green surfaces, with other subjects the effect does not occur (or becomes invisible).
My other HD 4K videos, taken some days before, also show that "flickering".
Did you also notice that problem with your Nano?
By the way: I like that "stars" sparkling in the water - this is no added video effect, it's just out of camera.

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