So, basically I encountered this bug in 21w16a but it is still in 21w17a. I don't know all the ways to get this bug, but this how I did.

1. I downloaded server.jar for 21w17a
2. I run it and I found good stuff like an Amethyst geode
3. I realize the caves and cliffs data pack is not there
4. I stop the server,put the datapack and re-run the server.
5. It started raining outside and the rain drops were inside the cave

P.S. this is how I encountered the bug, there might be another way.