It rains in patches instead of the all of the blocks.
It seems it only happens in some parts of the world
Environment
Intel i5 8600K - 16GB RAM - GTX1060 3GB - 4GB of memory dedicated to Minecraft - World imported from 20w16a
Linked issues
is duplicated by 12
MC-182504
Snow Golems don't generate snow layers
Resolved
MC-180085
Rain is patchy
Resolved
MC-180289
Rain missing from clear skies
Resolved
MC-180303
Rain missing in areas where it shouldn't
Resolved
MC-180340
Riptide during rain does not work in some places (may be dupe of MC-180303)
Resolved
7 more links
As far as I'm aware, worlds opened in 20w17a will continue to be affected by this issue. Optimizing the world will probably fix that for worlds opened in that version.