If the crouch button gets quickly tapped and released then often the screen will move up, then quickly down and back up again. (as if you are double tapping the crouch button really fast).

Update: Still an issue as of 1.16.5. The post this duplicates ( MC-152356 )and this post are said to be working as intended, but the reason as to why it's working as intended explains how it should work, not how the bug is said to be happening. In the video I attached, I crouched (pressed the crouch button) twice. The first time is an example of how it should work; the screen moved down when I pressed crouch and moved up when I released. The second crouch is the bug: I pressed and released crouch quick enough that the screen lowered, came up, lowered about halfway down (sometimes it lowers all the way), and came back up again, in a very short period of time. In the video it can be hard to notice, and when this happens in-game it is usually much more noticeable.