When going underground during thunderstorms during that period the rain particles travel through blocks and go all the way down to bedrock level.
The attacked image shows the water particles travelling down at the a y-Level 11
Environment
Windows 10 Java
this is the same bug as https://bugs.mojang.com/browse/MC-179858
worlds from before 20w17a have their heightmaps messed up when loaded in 20w17a. Can see with f3 the S and M values (for WORLD_SURFACE and MOTION_BLOCKING) are just completely random values. A world newly created with the new snapshot does not show this issue