The bug

Clouds are composed of squre cloud "blocks", which are put together to form larger clouds. The cloud blocks still have faces on all eight sides, and the faces inside of a cloud don't stop rendering. When looking at a cloud from outside (below or above), these inner faces are still somewhat visible (although barely noticeable) and create some ugly pixel fragments.

Original description

whilst playing I noticed what appears to be an aliasing issue in the clouds, usually they're just 'jaggy' near the sides but this is something else. and moving the mouse changed how it looked

I was running 20w16a 1920x1080@60, my screens native resolution is 3840x2160@60hz but I have my graphics card (GTX 1080 G1) is set to 1920x1080@60 to combat a lag issue in game

my processor is an i7-6700k @4.00ghz

Update

   After receiving a request to check whether or not it is still an issue in 1.16 rc1, I went back to check recreating the same conditions (changing my screen resolution to 1920x1080@60hz both in game and on my graphics card). I can confirm that the issue still persists.

https://imgur.com/gc4x6gg