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 MC-131770.
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.
Related tickets: MC-182789
Linked issues
relates to 15
MC-167103
Casting issue: Mob spawning loses precision at high coordinates
Resolved
MC-76810
Casting issue: Bone meal particles break at high coordinates (floating point precision)
Resolved
MC-161969
Casting issue: Adding ender eyes to end portal frames at high distances causes particles to lose precision
Resolved
MC-164352
Casting issue: Snow animation becomes stretched when being very high above the ground
Open
MC-167046
Casting issue: Lava ember particles and lava popping sounds lose precision on creation at high coordinates
Resolved
Remove the relates to for MC-76810