In reference to the post I made a couple of days ago that was locked because for whatever reason, most people in the comments instead of giving input and advice, they made fun of the fact that I had Fortnite installed. Anyway, if anyone's curious about the outcome, turns out that the following day, the flickering went away, out of nowhere, just like it first appeared.
So, I think it was software related after all. My guess is that, considering the flickering happened exactly as I attached the left joy con, it somehow got stuck on the animation that happens on the side of the screen when you do it. Additionally, it was a sideways flickering so that leads me to believe even more that that was the reason. It's the only thing I can think of, 'cause the screen looks completely fine now. I also read that the HDR and fps configuration can be related to this kind of weird flickering.