Since the game's original resolution is 400 x 240 (3DS res), it doesn't scale evenly to 1080p. 1080 ÷ 240 = 4.5. The result is a shimmering or flickering effect when scrolling horizontally. You can really notice it in the very first area in the woods. Look at how the background trees get dimmer while you move and brighter when you stop.
I didn't really think about that. I'm getting the Vita version, which would have the same issue, as it's 544p. It should be fine on the Switch's 720p screen, though, since that's a multiple of 240p.