I'm thinking this has something to do with Minecraft's recent implementation of OpenGL 3.2 or Java 16, because this issue does NOT occur in 1.16.5 or any previous version. Either that or something wrong with AMD drivers.

In this specific circumstance : when Clouds are set to "Fancy", during weather events (rain, snow), when over a large body of water, and when moving at high speeds (ex : Elyta + Fireworks), you'll start to notice these strange visual artifacts. All of the above criteria have to be met in order to experience this issue, otherwise your game will appear as normal. The artifacts appear and disappear at will in different parts of the world around me, and can take different shapes and colors depending on the surroundings. I've come to learn that these are called "Vertex explosions". This happens in vanilla with no resource packs applied. All screenshots were taken in vanilla, however the problem becomes infinitely more common with Optifine on. 

I've tried resetting my video settings, running a fresh install of the game with my settings unchanged, disabling any resource packs, all to no avail.

In the end, I've found that only two settings resolve the issue. 

1. Clouds set to "Fancy". When clouds are off or on "Fast", this issue will not occur.
2. High framerates. When capping the framerate to ~85 or lower (you might get away with a little higher), the issue will not occur. Important to note that my refresh rate is not at fault. Reducing the refresh rate to 60hz (I usually play on 144hz) in Windows display settings will not fix the issue. This only happens when capping the framerate in-game.

These two troubleshooting steps are independent. Either one will fix the problem, but not permanently as reverting your settings will reintroduce the issue. 

As framerate is very important to me, I've decided to disable my clouds temporarily until this issue is fixed or until I discover what is causing it. 

Steps to Reproduce:

1. Set your Clouds to "Fancy".
2. Set your framerate to "Unlimited", and disable Vsync.
3. Summon rain using commands (or wait for natural rain).
4. Locate a large body of water, such as an ocean.
5. Equip an Elytra + Fireworks and start moving at high speeds.

• It is important to note that this issue might be exclusive to AMD graphics cards. I've met several people who experience the same issues, all of them use AMD discrete GPUs running the latest optional drivers.
  • I've tested this on my laptop running a GTX 1050 with Nvidia's drivers, and have not seen these artifacts.