This is probably a dumb issue, but while playing around with Minecraft's settings, I found something called "Full Keyboard Gameplay". While using the arrow keys to look up and down, I decided to look all the way down to the ground, where then I realized pitch went below -90 (/ above 90).

(my username): pitch=91.422546386719

Nothing that should affect normal gameplay, but it could possibly cause false positives in anti-cheats resulting in a ban (like in my case).