https://wccftech.com/m1-ipad-pros-mini-led-display-might-not-be-perfect-for-reading-books-at-night/

While Mini-LED is great for watching HDR video and deeper blacks, but if you are going to read a book late at night, then be prepared to witness some halo-like effect.

Mini-LED on the 12.9-inch iPad Pro might not be perfect in one key area - reading. See, while an OLED display can turn off individual pixels completely to create a pitch-black effect in the tiniest of places with sheer finesse, such as the space inside the letter 'O', this is something which Mini-LED just cannot do at a pixel level.