When Saving any world, Minecraft tries to take a screenshot of sorts for the world preview. When RTX is enabled, one of the game's steps while saving and exiting is Disabling RTX. Because of this, when the world preview screenshot is taken, it's while the game is disabling the rendering of RTX leading to these fragmented world previews (attached).

Steps to Reproduce:

1. Get Any RTX resource pack
2. Load up any world with RTX Enabled 
3. Press Save and quit
4. Look at the world preview

I'm aware that this might be a duplicate, but for the life of me, I couldn't find this reported anywhere else...