The particles produced when rain hits the surface of blocks loses coordinate precision at high distances from spawn.
This issue is newly introduced by 20w06a - it is not at all present in 1.15.2 and particles behave normally there. May have been caused by the fix to
This is best tested when greater than 16,777,216 blocks out on both axes, so being 20 million blocks out should suffice. This issue is without a doubt due to incorrectly casting to a 32-bit float from an integer or double, resulting in numerical precision being lost. This can be fixed by simply not casting to a float, which eliminates any chance of losing any precision.
Rain rendering itself is also broken at very high coordinates on the xz plane, and has been for a while, but this is almost impossible to see in vanilla bounds.