OLEDs don’t suffer from the same kind of burn-in as other TV technologies, but they do something else, and it’s what I think has happened here. They’re made up of a very large number of sub-pixel cells, each containing an organic light-emitting substance that illuminates when an electric current is passed through it. They use a different substance for each of the RGB sub-pixels. These substances have a finite life span — they slowly break down over time as they are lit up, so gradually (typically over thousands of hours) they get dimmer.

I think you’ve run Zelda enough that the red pixels in the parts of your screen where the hearts are have taken a battering and started to noticeably dim. That’s why the heart area in your screenshot looks darker than the surrounding red area. On normal TV playback, all the different sub pixels are being worn out at the same rate, and you just get an imperceptible drop in brightness over a few years. But here you’ve displayed the exact same data (the game’s UI) for a very long amount of time and the wear is concentrated to that one specific area.

Phones with OLED displays usually subtly shift their persistent UI around by a few pixels eg. if there’s a clock shown on the lock screen. Again, this is about spreading the wear caused by that image over a larger area of screen.

There’s no fix for this, I’m afraid. It’s not like some forms of plasma or LCD retention that can be cleared with test patterns, because it’s actually a minor amount of damage to the screen itself.