There is a band that renders slightly before the rest of the screen and moves about depending on the angle of the camera.

When not in full screen mode the position of the band seems to stay the same regardless of the position of the window, so if I drag the Minecraft window to the top of the screen I can't see the flickering.

Seems to have been there since 1.14.3 pre 2

I tried to record it using a screen recorder but this actually fixed the issue until I stopped recording. At 00:13 you can kind of see it but it's much more obvious in game.